US troops have killed four passengers on a minibus in Baghdad's Sadr City district, say security sources in Iraq.
Survivors said there was no warning when troops opened fire
The troops opened fire during a raid in the impoverished suburb of the capital, which is dominated by a militia loyal to a radical anti-US Shia cleric.
Elements of Moqtada Sadr's Mehdi Army are suspected of capturing a US solider last month and kidnapping scores of hostages at a government office.
There has been no confirmation of the attack on the bus from the US military.
Five people were detained during the US-led raid, in which Iraqi forces participated, Iraqi sources said.
"I was surprised by the heavy shooting on our minivan. I was hit badly in my left hand," said a man at a Baghdad hospital.
"I can only feed my family when I work. What will happen now?"
Meanwhile, three US marines have been killed in western Iraq. The US military said the men died from wounds sustained in combat in Anbar province, where many Sunni Muslim insurgent groups are based.
The United Nations said on Wednesday that the number of Iraqi civilians killed in violence across the country had reached record levels.
The UN said the total number of killings in October was just over 3,700, almost 400 more than in the previous month.