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 a.m. (0330 GMT) 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 (ISAF) said the bomber killed two of its members and four civilian contractors. It declined to give their nationalities.

Concern about Afghanistan's prospects is growing as most foreign combat troops prepare to leave by the end of next year.

Karzai has accused neighboring Pakistan of meddling to de-stabilize his country. Analysts say Pakistan is maneuvering to limit the influence of its old rival, India, in Afghanistan.

Pakistan denies meddling and says it wants a peaceful and stable Afghanistan.