KABUL: A suicide bomber in a car attacked a convoy of foreign troops in Kabul on Thursday, killing at least 15 people including six Americans, Afghan and foreign officials said, in one of the worst attacks in the Afghan capital in months.

Forty people were wounded in the blast at around 8 am (0330 GMT) during the morning rush-hour. It caused heavy damage to mud-built houses in the vicinity.

The Hezb-e-Islami insurgent group, which is allied with the Taliban, claimed responsibility for the attack on the two-vehicle convoy.

Nato's International Security Assistance Force (Isaf) said the bomber killed two of its members and four civilian contractors. It declined to give nationalities.

But two senior officials, one Afghan and the other from Isaf, said the two Isaf soldiers and four contractors were all American.

Afghan officials said nine Afghan civilians were killed, including two children.

“Some of the dead civilians were badly burnt and can not be recognised,” Kaneshka Baktash, a spokesman for the Health Ministry, told Reuters.

Helicopters buzzed over Kabul's diplomatic area after the attack and sirens whined.

“We were in our home drinking tea when we heard a blast and our windows shattered, the glass wounded all of us,” Zohra, a wounded girl who only gave her first name, said from a hospital bed. Her head was wrapped in a bandage.

A Hezb-e-Islami spokesman told Reuters US military advisers were the targets.

“We planned this attack for over a week,” the spokesman, Haroon Zarghoun, said by telephone.

Last year, in a similar attack, the group killed seven South African and Russian pilots on their way to work in Kabul.

Hezb-e-Islami, which means Islamic Party, is a radical militant group which shares some of the anti-foreigner, anti-government aims of the Taliban.

But the political wing of the group, founded by warlord and former anti-Soviet fighter Gulbuddin Hekmatyar, has been in exploratory talks with Afghan President Hamid Karzai on a peace deal to end the 12-year war.

The National Directorate of Security, Afghanistan's intelligence agency, says it thwarts a large number of attacks on the capital on a weekly basis.

The last suicide bomb attack in Kabul was in March, when a man blew himself up at a Defence Ministry gate, killing nine Afghans, during a visit by US Defence Secretary Chuck Hagel.