A suicide bomber rammed an explosives-rigged car into a military convoy on Tuesday, killing three American soldiers and three Afghan civilians in eastern Afghanistan.

The attack against the American convoy came in eastern Kapisa province, an stronghold of insurgents loyal to the Afghan warlord Gulbuddin Hekmatyar.

"I was driving my motorbike when I saw the car with a young man with a beard and white cap," said Sayed Najibullah, a 22-year-old shopkeeper.

Najibullah said he heard the explosion minutes after the man, in a Toyota Corolla, waved him past.

Three U.S. troops were killed in the explosion, said Tech. Sgt. Chuck Marsh, a U.S. military spokesman. The soldiers served with NATO''s International Security Assistance Force.

Three civilians also died and two others were wounded, the Interior Ministry said in a statement.

Taliban regularly use suicide attackers and roadside bombs in assaults on foreign and Afghan troops across the country. Such attacks were up 25 percent the first four months of 2009 compared with the same period last year.