KABUL - 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 spokesperson. 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 per cent the first four months of 2009 compared with the same period last year.

Bomb attacks will rise 50 per cent this year to 5,700 -- up from 3,800 last year, U.S. military officials predict.

According to military figures, 172 coalition forces were killed in such attacks last year -- and far more Afghan civilians died.

In the eastern Logar province, meanwhile, U.S. and Afghan troops called in airstrikes on two groups of militants, killing 13 insurgents Tuesday, the U.S military statement said.

Separately, in the eastern Khost province, a convoy of Afghan and American troops killed the driver of a car when the vehicle did not slow down in response to shouts to stop and warning shots, said Chief Petty Officer Brian Naranjo, a U.S. forces spokesperson.

"They fired to stop the vehicle and killed the driver," Naranjo said.

In the south, U.S. forces said they killed eight Taliban fighters in a clash Uruzgan province on Monday. The coalition said two of its troops and three Afghan policemen were wounded during the clash.

They were undergoing medical treatment and were in stable condition. The troops were on patrol when Taliban fighters attacked with small-arms fire and heavy machine-guns.

Southern Afghanistan is the centre of the Taliban-led insurgency, where thousands of new American troops will join the fight this year.

President Barack Obama hopes the new troops can turn the tide of the Taliban successes in the last three years.