A bomb attack on US troops north of Kabul on Tuesday killed three soldiers and three Afghan civilians, military and Afghan officials told AFP.
The attack was carried out near Bagram, a town about 50 kilometres (30 miles) north of Kabul where the United States has its largest military base in Afghanistan, officials said.
"Three International Security Assistance Force (ISAF) service members were killed and one injured after an improvised explosive device attack in eastern Afghanistan this morning," ISAF said in a statement.
The US military and another ISAF official said separately that the bomb had hit US troops. The ISAF official had said his first information was that four US soldiers had died.
The military could not confirm Afghan reports that it was a suicide attack.
Afghan interior ministry spokesman Zemarai Bashary told AFP: "A suicide car bombing has happened and the target was an international forces convoy.
"The initial report we have received says three civilians were killed and three wounded."
The civilians were just passers-by and it was not yet clear who they were, he said.
There was no immediate claim of responsibility, but the Pakistan-based Afghan Islamic Press news agency reported that the Hezb-e-Islami faction led by former prime minister Gulbuddin Hekmatyar said it had carried out the attack.
The faction is one of several groups involved in the insurgency in Afghanistan, including the Taliban who were in government between 1996 and 2001.
Two US nationals -- a woman soldier and a male civilian contractor -- were killed just south of Bagram on Wednesday last week when a remote-controlled bomb was detonated to hit their convoy.
ISAF comprises around 58,000 soldiers drawn from around 40 countries sent to Afghanistan to fight an insurgency led by the Taliban, and help the Afghan government establish its authority.
Most of the soldiers are from the US military, which also has troops in a separate deployment called Operation Enduring Freedom that led the invasion that removed the Taliban from power.