Two U.S. helicopters have crashed in northern Iraq, killing four U.S. soldiers in the first such incident this year.

It is not entirely clear what caused the early morning crash, but a U.S. military spokesman in Baghdad, Major Jose Lopez, said that "enemy fire" was not the suspected cause.

"The cause of the incident is unknown and it is under investigation.  We are waiting for more information to be released and the names of the deceased are being withheld pending notification of next of kin and released by the Department of Defense," said Lopez.  "The cause, although it is unclear, does not appear to be by enemy action."

Iraqi military sources pinpointed the site of the crash near the religiously mixed city of Kirkuk, which remains one of the more restive areas of Iraq.

In previous recent incidents, a U.S. OH-58 helicopter crashed on November 15 near the northern Iraqi city of Mosul, while two UH-60 "Blackhawk" helicopters crashed while landing in Baghdad last October.

A civilian cargo plane also crashed in November, killing all seven people on board.  In 2007, the crash of a transport helicopter in northern Iraq killed seven soldiers.

U.S. "Blackhawk" helicopters normally fly in pairs, and each reportedly has a crew of two pilots and two gunners.