Four U.S. soldiers killed in helicopter crash in northern Iraq

"Four coalition forces members were killed when two aircraft went down in northern Iraq at approximately 2:15 a.m.," U.S. military spokesman Major Jose Lopez said.

"The cause of the incident is unknown and is under investigation," he was quoted by Reuters as saying.

A police official told AFP the helicopters collided near Kirkuk, 255 kilometers (158 miles) north of the capital Baghdad.

The U.S. military is currently taking a back seat to an increasingly large Iraqi force made up of 560,000 policemen and 260,000 military personnel, with the U.S. providing military logistical and air support on request.

Air crashes have remained common in Iraq although while they have waned. In November, a civilian cargo plane crashed in western Iraq killing all seven people on board.

Seven soldiers were killed when a Chinook transport helicopter crashed in southern Iraq in September.