US troops have shot dead two Iraqis on the outskirts of Falluja, a flashpoint town west of Baghdad, coalition officials say.
A statement said troops shot at a van after coming under fire at a checkpoint late on Friday night.
But local reports and eyewitnesses said four people, all of them civilians, had been killed in the incident.
Two women were among the dead, sources said.
There were reports of an angry anti-US demonstration in the area as news of the shooting spread.
The US military said an investigation would be carried out into the incident.
Falluja, 50 kilometres (30 miles) from Baghdad, is a stronghold of supporters of ousted Iraqi dictator Saddam Hussein.
It has seen repeated clashes involving US troops.
Correspondents say anti-US resistance has been strong there since American troops killed 16 demonstrators in April.
Earlier this month they killed a 14-year-old boy when they mistook a wedding celebration for a hostile attack.
Rules of engagement
On 12 September US troops killed eight local security officers in Falluja.
US troops shot Iraqi policemen in Falluja on 12 September
The US military says its initial findings indicate that on that occasion the troops acted in line with their rules of engagement.
Iraqi policemen who survived the incident said the firing began as several Iraqi police vehicles approached a US checkpoint on the outskirts of Falluja.
The police, all members of the local US-trained force, were chasing a car carrying several wanted gunmen, they said.
The surviving policemen said they had begged the American soldiers to stop shooting, screaming in Arabic and English that they were police officers.
But the troops kept firing for between 30 minutes and an hour, they added.