By Atia Abawi and Fazal Ahad, NBC News

KABUL, Afghanistan -- Six Americans were killed when a suicide bomber targeted a convoy carrying foreign troops in Kabul on Thursday, according to a NATO source.

The victims included two soldiers and four civilian contractors, the source added.

An official said six Afghan civilians had been killed, also, Reuters reported, adding that the Hezb-e-Islami insurgent group, allied with the Taliban, claimed responsibility for the attack. A spokesman for the militants told Reuters that U.S. military advisors were the targets.

Kabul police spokesman Hashmatullah Stanikzai said the attacker detonated a Toyota Corolla car. He added that six people had been killed and 35 others injured.

The powerful blast took place at around 8 a.m. local time (11.30 p.m. ET Wednesday).

"It was a powerful explosion and some of the dead civilians were badly burned and cannot be recognized," Kane Backlash, a spokesman for Afghanistan's Health Ministry, told Reuters.