BAGHDAD — An American military official said a mechanical problem appeared to be the reason for a helicopter crash yesterday 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 yesterday’s crash highlighted the noncombat dangers facing Americans in Iraq.

In all, 70 American 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.

Major John Hall, a military spokesman in Baghdad, said hostile fire had been ruled out in yesterday’s crash and that the three other helicopters suffered no damage.

A Pentagon official in Washington said it appeared that the twin-engine transport aircraft had malfunctioned.

“They think it was a mechanical problem,” the official said on condition of anonymity for lack of authorization to speak to the press on the record.

The Chinook, considered the Army’s workhouse, has the capacity to carry more than 30 people, but the military said the seven killed were the only ones on board the helicopter that crashed.