A U.S. helicopter airstrike on Wednesday night killed eight civilians, including two children, north of Baghdad, police officials said on Thursday.

Colonel Mudhher al-Qaisi, police chief in the town of Baiji, said the attack was on a group of shepherds in a vehicle in a farming area. Relatives said some of those killed were fleeing on foot after the U.S. military arrived in the area.

"This is a criminal act. It will make the relations between Iraqi citizens and the U.S. forces tense. This will negatively affect security improvements," Qaisi told Reuters.

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A U.S. military spokeswoman, Lieutenant-Colonel Maura Gillen, said the helicopter fired on the vehicle after observing "suspicious activity". She said the driver had ignored warnings to stop.

The incident is the latest in a string of U.S. airstrikes in which civilians have been killed. It comes at a bad time for the U.S. military, which has been working hard to soothe tensions with Prime Minister Nuri al-Maliki's government over the shooting of a copy of the Koran, the Muslim holy book, by a U.S. soldier earlier this month.

United Nations officials have expressed concern at the number of civilians killed in airstrikes in Iraq and said more care must be taken in military operations to protect them.

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