Awakening Council members at the site of the air strike near Samarra
Six people have been killed in a US air strike near the Iraqi town of Samarra, with some reports suggesting they were US-allied anti-al-Qaeda Sunni fighters.
The US denied claims by a police source and a militia member that those killed at the checkpoint were members of an Awakening Council.
The US-funded groups are credited with helping to curb the level of violence.
It came as four more US soldiers were killed in Iraq, bringing the death toll since the 2003 invasion close to 4,000.
Three soldiers died when a roadside bomb hit their vehicle near Baghdad on Saturday, the US military said.
Another US soldier died after a rocket or mortar attack on Friday.
A US helicopter gunship killed the six Iraqis near Samarra in an attack which an army spokesman said had been launched after people "were spotted conducting suspicious terrorist activity".
The area, 125km (80 miles) north of Baghdad, has seen a large number of roadside bombings.
The US said it was now investigating the attack, in which two other people were injured.
But Abu Faruq, a local leader of an Awakening Council in Samarra, said those killed were members of his group, which was manning a checkpoint. He told the AFP news agency that they had all been wearing the reflective vests worn by members of the Councils.
A BBC correspondent in Iraq says some dissidents are believed to be continuing their support for al-Qaeda while pretending allegiance to the Awakening Councils.
Independent website icasualties.org - which keeps a count of US deaths in Iraq - says Saturday's American fatalities would bring the death toll to 3,996.
Iraq has been marking the fifth anniversary of the invasion
Iraq this week marked the fifth anniversary of the US-led invasion.
US President George W Bush said the invasion had been "the right decision" and had made the world better.
He said the US military's co-operation with Sunni Arab militias was yielding the first large-scale Arab uprising against Osama Bin Laden, and that last year's US troop surge had opened the door to a major strategic victory.
Iraqi President Jalal Talabani said the "liberation of Iraq" by US-led forces had been the start of a new era, but he also warned that today's Iraq was still gravely threatened.
The campaign group, Iraq Body Count, says the civilian death toll since March 2003 is between 82,000 and 89,000, although it warns many deaths may have gone unreported.