A suicide bomber in a car attacked a convoy of foreign troops in Kabul today, killing at least 15 people including six Americans, Afghan and foreign officials said.

It was one of the worst attacks in the Afghan capital in months.

About 40 people were wounded in the blast during the morning rush-hour.

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 said the bomber killed two of its members and four civilian contractors.

It declined to give their 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 cannot be recognised," Kaneshka Baktash, a spokesman for the Health Ministry, said.

Afghan President Hamid Karzai strongly condemned the "cowardly" attack.

"Terrorists and enemies of Afghanistan's peace brutally targeted a residential area," Mr Karzai said in a statement.