At least fourteen people were killed in a suicide bomber attack targeting a NATO convoy in Kabul on Thursday. 

The victims included six Americans, two of whom were soldiers and four were civilian contractors, NATO sources told NBC News. Reuters reported that two senior officials, one Afghan and the other from ISAF, said the two soldiers and four contractors were all American.

However, the final tally on the number of casualties and their nationalities have varied across news reports. The Nato-led International Security Assistance Force (ISAF) said two of its soldiers and four civilian contractors died. Meanwhile, police at the scene told CBS News that at least five U.S. advisers and one of their interpreters were killed, in the attack, but there was no confirmation from the officials. Afghan officials initially said that all six killed were civilian bystanders and the dead included two children.

The blast took place at about 8 a.m. local time (03:30 GMT) in the industrial district of Karta-e Naw, east of Kabul city, during morning rush hour.

"A [Toyota] Corolla suicide car detonated near two foreign military vehicles," Kabul police Chief Mohammad Ayoub Salangi told the AFP news agency on Thursday. "There are casualties. About 10 houses were severely damaged."

Kaneshka Baktash, a spokesman for Afghanistan's Health Ministry, told Reuters: 

"It was a powerful explosion and some of the dead civilians were badly burnt and cannot be recognized," 

A spokesperson for the Hizb-e-Islami insurgent group claimed responsibility for the blast and said that U.S. military troops were the targets.

"We planned this attack for over a week, our target was American advisers," Hizb-e-Islami spokesman Haroon Zarghoun told Reuters by telephone, adding that the bomb killed 12 Americans, the news agency reported.

The last major attack in Kabul was on Mar. 9, when a suicide bomber blew himself up near the defense ministry killing nine Afghans, during a visit of U.S. Defense Secretary Chuck Hagel to the city. At least 15 foreign troops have been killed in separate incidents in Afghanistan since the end of April when the Taliban launched its spring offensive.