Six Americans were among at least 15 people killed when a suicide bomber targeted a convoy carrying foreign troops Thursday in Afghanistan's capital city of Kabul, a NATO source told NBC News. The American victims included two soldiers and four civilian contractors, the source added. Afghan officials said two children were among the Afghan victims. Forty others were injured in the blast after the attacker detonated a Toyota Corolla car, Kabul police spokesman Hashmatullah Stanikzai said. Hizb-i-Islami, an insurgent group which is allied with the Taliban, claimed responsibility for the attack.