Baghdad ? A U.S. military official said a mechanical problem appeared to be the reason for a helicopter crash Thursday that killed seven American soldiers in Iraq’s southern desert, the deadliest such incident in Iraq in more than a year.

The CH-47 Chinook was flying with three other choppers from Kuwait when it went down shortly after midnight about 60 miles west of Basra, the military said.

The U.S. military relies heavily on helicopters to ferry troops, dignitaries and supplies to avoid the threat of ambushes and roadside bombs, and Thursday’s crash highlighted the noncombat dangers facing Americans in Iraq.

In all, 70 U.S. helicopters have gone down since the war started in March 2003, according to military figures. Of those, 36 were confirmed to have been shot down.