Afghan and U.S. military officials said three American soldiers and three Afghan civilians have been killed in a suicide car bomb attack north of Kabul.
Authorities said the blast occurred Tuesday when the driver of an explosives-laden Toyota Corolla rammed the vehicle into a NATO convoy.
The Reuters news agency quoted a spokesman for Afghan warlord Gulbuddin Hekmatyar as saying his militant group Hezb-i-Islami had planned the attack.
Elsewhere, the U.S. military said it killed 13 militants during airstrikes Tuesday in eastern Logar province.
Separately, in eastern Khost province, a convoy of Afghan and American troops killed a civilian it feared was a suicide bomber after a car approached the convoy.
In the south, U.S. military forces said they killed eight Taliban militants during a clash in Uruzgan province on Monday.
U.S. President Barack Obama has ordered 21,000 additional troops to reinforce the 38,000 U.S. troops already in Afghanistan.
The New York Times said U.S. Defense Secretary Robert Gates has said he believes U.S. support for the war in Afghanistan will evaporate in less than a year unless President Barack Obama's administration achieves a "perceptible shift in momentum."
Gates said Taliban militants currently have the momentum in Afghanistan.
Some information for this report was provided by AP and Reuters.