This was CNBC's live blog tracking developments on the war in Ukraine. See here for the latest updates.

Top Russian officials, including President Vladimir Putin, continue to make baseless claims that Ukraine, as well as its Western allies, were involved in a deadly terrorist attack in Moscow last Friday.

The head of Russia's Federal Security Service (FSB) on Tuesday said that the U.S., U.K. and Ukraine were behind the attack in which 139 people died.

FSB chief Alexander Bortnikov told pro-Kremlin journalist Pavel Zarubin that the attack was beneficial to Western intelligence services and Ukraine in their bid to destabilize Russia. He did not present evidence to back up his claim.

The Islamic State militant group said it was behind the attack. Ukraine, meanwhile, has repeatedly and vehemently denied any involvement. The White House has said Russian claims of Ukrainian involvement were "Kremlin propaganda." The U.K., U.S. and Ukraine have not commented on Bortnikov's latest claims.

On Monday, President Vladimir Putin conceded that the deadly terrorist attack in Moscow last Friday was carried out by "radical Islamists" but insisted that there was a link to Ukraine, without presenting evidence.

Russia sees no chance of Swiss leading Ukraine peace process, envoy says

Russia sees no chance of Switzerland leading efforts to secure peace in Ukraine, a senior Russian diplomat said on Tuesday, after Bern said it planned to host a high-level Ukraine peace conference in the coming months.

Swiss authorities have said that Russia is unlikely to take part, at least not at the outset of the talks.

"As of now, we don't see any possibility that Switzerland would take the lead and organise something," said Gennady Gatilov, Russia's Permanent Representative to the United Nations in Geneva.

Russia, which launched a full-scale invasion of Ukraine two years ago, has said the Swiss initiative is doomed to fail without Moscow's participation.

Gatilov said that while Moscow was not against negotiations to end the war, it would not take part in talks in Switzerland, a country he said had relinquished its neutrality with its stance on the conflict.

Switzerland has adopted the European Union's sanctions against Russia over the invasion and frozen some 7.7 billion Swiss francs ($8.53 billion) in financial assets belonging to Russians, which Gatilov described as "stolen money".

"That's why we believe that Bern unfortunately devaluated its status as a neutral state," he said.

— Reuters

Ukraine's Zelenskyy dismisses head of security council

Ukrainian President Volodymyr Zelenskyy on Tuesday dismissed the secretary of the country's security council, Oleksiy Danilov, according to a Google-translated decree published on the government website.

Danilov has held the position since October 2019. His position will now be held by Oleksandr Lytvynenko, who served as the head of the Ukrainian foreign intelligence service.

No reason was given for the decision, which comes shortly after Zelenskyy replaced the head of the armed forces in a key military shake up.

— Sophie Kiderlin

Russian intelligence chief claims U.S., U.K. and Ukraine behind Moscow attacks

The head of Russia's Federal Security Service (FSB) on Tuesday said that the U.S., U.K. and Ukraine were behind a deadly terrorist attack in Moscow last week.

FSB chief Alexander Bortnikov told pro-Kremlin journalist Pavel Zarubin claimed that the U.S., U.K. and Ukraine were responsible for the attack, claiming that the attack was beneficial to Western intelligence services and Ukraine to destabilize Russia.

"We believe that the action was prepared by the radical Islamists themselves, and naturally the Western intelligence services contributed to this, and the Ukrainian intelligence services themselves are directly related to this," Bortnikov said, RIA Novosti reported.

Ukraine has already vehemently denied any involvement in the attack and the White House has said Russian claims to the contrary are "Kremlin propaganda." The U.K., U.S. and Ukraine have not commented on Bortnikov's latest claims.

Almost 140 people were killed in the Crocus City Hall concert hall, when gunmen entered the venue and opened fire, as well as set fire to the venue. The Islamic State militant group claimed responsibility, but Russia was quick to connect Ukraine to the outrage, without presenting evidence.

Since then, Russian President Vladimir Putin has accepted that the attack was carried out by so-called "radical Islamists," but continued to claim Ukraine was linked to the attack.

Bortnikov reiterated Russia's claim that the attackers were caught, as they tried to flee toward Ukraine, where, he said, they were expected and were going to be greeted "as heroes."

"The bandits intended to go abroad. Precisely to the territory of Ukraine. According to our preliminary operational information, they were expected there," he told Zarubin, in an interview posted in Russian on his Telegram channel.

— Holly Ellyatt

Russia's Shoigu says idea of further mobilization is 'nonsense'

Russian Defense Ministry Sergei Shoigu, asked on Tuesday whether an additional wave of mobilization would be required in order to secure a buffer zone protecting Russian territory from Ukrainian attack, called the idea "nonsense", the state news agency RIA reported.

Russian President Vladimir Putin said this month that Moscow may need to occupy a buffer zone in order to protect the Belgorod region, which has in recent months come under heavy attack from Kyiv's forces across the border in Ukraine.

— Reuters

One of Russia's most powerful officials backs baseless claim of Ukraine involvement in attack

One of the most powerful and influential men in Russian President Vladimir Putin's inner circle claimed unequivocally on Tuesday that Ukraine was responsible for the Moscow terrorist attack carried out last Friday.

Nikolai Patrushev — the secretary of Russia's Security Council and in charge of issuing guidance and policy proposals on national security issues — was asked by Russian reporters whether Ukraine or the Islamic State militant group was behind the attack that killed 139 people. The Islamic State group said it had carried out the attack.

"Of course Ukraine," Patrushev answered reporters, according to a Google-translated article from news agency RIA Novosti.

A former intelligence officer and an ideological Kremlin figurehead who is extremely close to Putin, Patrushev joins the ranks of several high-profile Russian officials pointing the finger of blame at Ukraine without presenting any evidence of Kyiv's involvement.

Ukraine itself denies any role in the attack. President Volodymyr Zelenskyy said it was "absolutely predictable" that Moscow would try to pin the blame on Ukraine.

Patrushev's comments could signal that Moscow is ready to double down on its allegations of a link between Kyiv and the terror perpetrators. Analysts say the Kremlin is likely to use the attack to its advantage, politically, in any case, perhaps to put Russian citizens on a firmer war footing and ahead of possible further mobilization.

So far, the Kremlin has been cautious about giving further details or evidence to back up its claims of a connection. Putin's press secretary refused to be drawn on the matter earlier on Tuesday.

On Monday, Putin conceded that the attack had been carried out by "radical Islamists," but once again claimed there was a link with Ukraine, again without presenting proof.

— Holly Ellyatt

Kremlin refuses to comment on alleged link between Moscow attackers and Ukraine

The Kremlin refused to comment Tuesday on whether it believed there was a link between the Ukrainian government and the gunmen who killed 139 people in a terrorist attack on a Moscow concert hall last Friday.

Asked during a call with reporters whether there was a direct link between Ukraine and the "radical Islamists" that were behind the attack, as President Vladimir Putin described the suspects on Monday, Kremlin Press Secretary Dmitry Peskov said he had "nothing to add to what has already been said on this topic."

"As for whether it is possible to say 'one way or another': you can say it any way you want. And observers and analysts can probably afford this. But (...) while the investigation is underway, the official authorities cannot afford to do any statements on this matter," Peskov told reporters, news agency Interfax reported.

"Although I recommend that you very carefully reread President Putin's statements that he made over the last two days. In this context, they are very important," Peskov said.

During a conference call with Kremlin officials on Monday, President Putin said the attack was carried out by "radical Islamists" but again claimed that there was a link to Ukraine, or a "Kyiv trace," as Moscow has described it and that U.S. intelligence dismissing any link was unconvincing.

Russia has presented no evidence that there was any involvement by Ukraine and Kyiv itself vehemently denies any role in the deadly attack on concertgoers.

Eight suspects, nationals of Kyrgyzstan and Tajikistan, have been remanded in custody and charged with terrorism offenses ahead of trial.

—  Holly Ellyatt

Russia's FSB claims it has prevented another terrorist attack in Russia

Russia's security service, the FSB, reportedly announced on Tuesday that it had prevented a terrorist attack at a humanitarian aid reception point in the Samara region in southwestern Russia, news agency Interfax reported.

The FSB claimed the attack had been planned by an accomplice of the "Russian Volunteer Corps," an anti-Kremlin Russian rebel group that has conducted cross-border raids into Russia during the war. Russian rebel groups have been labeled as terrorist organizations in Russia and are banned.

"A terrorist act planned by a citizen of the Russian Federation, born in 1994, an accomplice of the terrorist organization Russian Volunteer Corps (RDK), operating under the patronage of the Ukrainian special services, was prevented on the territory of the Samara region," the intelligence service said, according to Interfax.

The FSB reported that the suspect was in possession of an improvised explosive device and planned "to detonate it in the building of a humanitarian aid reception point for one of the volunteer organizations." It did not provide evidence for the claim.

The FSB said the suspect had detonated the device during his arrest and he had been killed. CNBC was unable to verify the claims in the report.

— Holly Ellyatt

Russian investigators query families of shooting suspects in Tajikistan, sources say

Russian investigators were in Tajikistan on Tuesday, questioning the families of four men charged with carrying out a deadly attack on a concert hall near Moscow, three Tajik security sources told Reuters.

The sources, who were not authorized to comment publicly, said Tajik security officials had brought the families to the capital of Dushanbe from the towns of Vakhdat and Gissar, and from the Rudaki district.

Tajik President Emomali Rakhmon was personally overseeing the investigation on the Tajik side, the sources said.

On Monday, making his first public comment on the attack, Rakhmon called it a "shameful and terrible event" and urged Tajiks to protect their children from harmful influences.

Four men of Tajik origin have been remanded in custody on terrorism charges, on suspicion of carrying out the attack. Three others, also of Tajik origin, were remanded on suspicion of complicity.

Islamic State has said it was responsible for the attack and has released video footage that it says shows the massacre.

— Reuters

Ukraine says it has hit a warship that Russia seized from it in 2014

Ukraine says it hit a landing ship, the Konstantin Olshansky, that Russia had captured from it in 2014.

"Currently, this ship is not combat-capable," Ukrainian navy spokesperson Dmytro Pletenchuk said on national television, according to comments reported by Reuters.

"It had gone through a renovation and was being prepared for use against Ukraine, so unfortunately the decision was taken to strike this (ship)," Pletenchuk added. He said a Ukrainian-made Neptune anti-ship missile was used in the latest strike.

Russia seized the Konstantin Olshansky from Ukraine when its forces invaded Crimea in 2014.

Ukraine has carried out a series of successful drone and missile strikes against Russia's Black Sea Fleet, forcing Russia to withdraw a number of its warships from the Crimean peninsula, and its headquarters in Sevastopol.

On Sunday, Ukraine's military said defense forces had successfully hit two large landing ships in Sevastopol.

— Holly Ellyatt

Putin says 'Islamist radicals' carried out terror attack, but insists on Ukraine link

Russian President Vladimir Putin conceded on Monday that the deadly terrorist attack in Moscow last Friday was carried out by "radical Islamists" but insisted that there was a link to Ukraine, without presenting evidence.

Putin told senior Kremlin officials Monday that an investigation into the attack on the Crocus City Hall concert venue, in which 139 people died when gunmen entered the building and shot indiscriminately, "must be carried out in the most professional manner, objectively, without any political bias."

The Islamic State group said it was behind the attack on Friday and published footage of it. Four men appeared in a Moscow court on Sunday night and were charged with terrorism offenses. Three more men were charged Monday.

Putin initially suggested the attack was linked to Ukraine, claiming that the suspects had fled toward the border and that "a window had been prepared for them on the Ukrainian side to cross the border." He did not present any evidence for the claim. Kyiv denies any involvement.

On Monday, Putin appeared to accept that the IS group was responsible for the attack but insisted that there was a link to Ukraine.

"We know that the crime was committed by radical Islamists, whose ideology the Islamic world itself has been fighting for centuries," Putin said, according to comments translated by NBC.

"But we also see that the United States, through various channels, is trying to convince its satellites and other countries of the world that, according to their intelligence data, there is supposedly no Kyiv trace in the Moscow terrorist attack, that the bloody terrorist attack was carried out by followers of Islam, members of the ISIS organization banned in Russia."

U.S. officials have dismissed any Ukrainian involvement in the attack, with White House spokesperson John Kirby telling reporters, "There was no linkage to Ukraine. ... This is just more Kremlin propaganda."

— Holly Ellyatt

Russian refineries should expect more 'more blasts,' Ukraine security service head says

The head of Ukraine' Security Service (SBU) said more "special operations" will be carried out this year as Ukraine looks to inflict more damage on Russian military hardware and infrastructure.

"The Russians should expect more blasts ... We never repeat ourselves and if they change their defense system against our naval drones, we, of course, are one step ahead. Everything needs to be done in the right time, you will see how it goes," Vasyl Maliuk said in an interview with ICTV that was reported by news agency Ukrinform.

Maliuk claimed Ukrainian security agencies have destroyed 809 Russian tanks, as well as other armored vehicles and e-warfare systems since the start of the war. He also said the security service was operating attack drones both against Russian front-line positions and within Russia itself. Ukraine has had success attacking Russia's Black Sea Fleet around Crimea as well as its energy industry.

The SBU has attacked 13 oil refineries across different regions of Russia in a bid to deplete Russia's energy export revenues that fund its war machine. Maliuk said Russia's refining capabilities had been reduced by 12%; British intelligence also suggests that at least 10% of Russia's refining capacity has been disrupted by Ukrainian attacks.

When asked whether Russian refineries will continue to burn, Maliuk replied, "Of course, they should expect more blasts."

— Holly Ellyatt

U.S. dismisses Russia linking shooting to Ukraine as "Kremlin propaganda"

The White House on Monday dismissed Russian claims that the shooting attack that killed 137 people in a concert hall outside Moscow was linked to Ukraine.

"There was no linkage to Ukraine. ... This is just more Kremlin propaganda," White House spokesman John Kirby told reporters in a briefing.

— Reuters

Moscow attackers 'deserve no mercy,' Russian prime minister says, amid debate over death penalty

Russia's prime minister told officials Monday that those responsible for the Crocus City Hall terrorist attack on Friday deserved to be shown "no mercy."

Prime Minister Mikhail Mishustin was addressing Communist Party lawmakers Monday, when he commented on the deadly shooting, which he described as a "monstrous terrorist act."

"These events are now being investigated. Those responsible will be punished. They do not deserve mercy," the prime minister said, according to news agency Interfax.

The Islamic State group said it was behind the attack on Friday, which left 137 people dead and 182 injured.

Four men suspected of carrying out the attack were charged with terrorism at a pre-trial hearing in a Moscow court on Sunday evening. Two of the suspects admitted their guilt in the attack, the court said on Telegram, according to a Google translation.

A debate is raging in Russia over whether a moratorium on the death penalty should be lifted, with several prominent lawmakers reportedly arguing for the reintroduction of the punishment for terrorism convictions.

Earlier, Kremlin Press Secretary Dmitry Peskov told reporters that the Kremlin was "not currently taking part in this discussion."

— Holly Ellyatt

Parents of detained Wall Street Journal reporter optimistic over talks for his release

The parents of Wall Street Journal reporter Evan Gershkovich have expressed optimism over the state of government-level talks over his release, as he approaches his one-year anniversary in Russian captivity.

"We have president Biden's promise, delivered to us personally and in the State of the Union [address], and we know that the U.S. government is taking the case very seriously, so we are optimistic," Gershkovich's mother, Ella Milman, said during a televised ABC News interview.

Gershkovich's father, Mikhail, added they were "happy that both governments have expressed willingness to negotiate."

Gershkovich was apprehended by Russian authorities on March 29, 2023, while on assignment in Yekaterinburg. He has been accused, including by Russian President Vladimir Putin, on charges of espionage – which his employer, the Wall Street Journal, vehemently denies.

Putin has previously signaled the possibility of swapping Gershkovich for a Russian prisoner detained abroad, but an agreement has yet to be reached.

 — Ruxandra Iordache

Moscow attack suspects appear in court

Four men Russia says were involved in the deadly shootings at a concert hall on the outskirts of Moscow Friday night appeared in court late on Sunday.

The men, three of whom confirmed as Tajik nationals, were charged with committing an act of terrorism when they appeared in the Basmanny district court late last night. They're accused of carrying out an attack in the Crocus City Hall concert venue in which 137 concertgoers were killed and at least 140 were injured.

The suspects, identified as Saidakrami Murodali Rachabalizoda, Dalerdzhon Barotovich Mirzoyev, Shamsidin Fariduni and Muhammadsobir Fayzov, will be detained in custody until May 22, pending trial, the court said on Telegram. The suspects may face life in prison if found guilty, the RIA Novosti news agency reported.

One of the suspects was brought into court in a wheelchair while another had a bandage removed from his face, revealing a black eye. Another appeared dazed and disoriented, video footage of the suspects released by the court showed.

— Holly Ellyatt

After deadly attack, what could Putin do next?

Russian President Vladimir Putin's next move is being watched closely after he linked Ukraine to the deadly terrorist attack that took place in a Moscow concert hall Friday, leaving at least 137 people dead.

Russian officials and pro-Kremlin hawks continue to level blame at Kyiv for the attack at the Crocus City Hall venue, where gunmen killed concert-goers, including three children, and wounded at least 140.

Kyiv vehemently denies any involvement and Russian authorities are investigating the attack.

Political analysts say that whatever the outcome of that investigation, Putin is likely to use the tragedy to his advantage as he looks to boost domestic support for the war in Ukraine and to "advance its broader geopolitical objectives," as one analyst put it.

Read more on the story here: Putin expected to use deadly Moscow attack to Russia’s advantage, whoever’s to blame

Read CNBC's previous live coverage here:

Moscow terror attack suspects appear in court beaten and bruised; Russia warned against blaming Kyiv