BAGHDAD, Iraq - A U.S. Marine helicopter transporting troops crashed today in the desert of western Iraq, killing 31 Marines, American military officials said. It was the deadliest crash of a U.S. military helicopter in Iraq.

A Pentagon source said the helicopter was a CH-53 Sea Stallion, which is normally configured to carry 37 passengers, but can take up to 55.

A Bush administration official said there were no survivors from the crash and that the cause was not immediately known.

A search and rescue team has reached the site and an investigation into what caused the crash was under way. The administration official said there was bad weather at the time.

President Bush planned a statement.

"We are saddened anytime there is loss of life of our troops in harm's way," White House press secretary Scott McClellan said.

The death toll was the largest for a U.S. aircraft crash in Iraq since U.S. forces invaded in March 2003.

The helicopter went down about 1:20 a.m. near the town of Rutbah, about 220 miles west of Baghdad, while conducting security operations, the military said in a statement. The aircraft was transporting personnel from the 1st Marine Division.

Previously, the deadliest incident involving U.S. troops was a Nov. 15, 2003, crash of two Black Hawk helicopters that collided while trying to avoid ground fire in Mosul, killing 17 U.S. soldiers and wounding five.

Earlier that month, a Chinook transport helicopter was shot down by a shoulder-fired anti-aircraft missile near Fallujah, killing 16 American soldiers and wounding 26.

The military has lost at least 33 helicopters since the start of the war, including at least 20 brought down by hostile fire, according to a study by the Brookings Institution.

The deadliest day for American troops during the initial invasion of Iraq was March 23, 2003, when 26 Americans were killed in a number of separate incidents. President Bush declared major combat over on May 1, 2003, but fighting has continued.

Last month, a suicide bomb exploded at a mess tent in a base near Mosul, killing 22 people including 14 U.S. soldiers and three American contractors.

Meanwhile, insurgents carried out a string of five car bombs across the country today, including three that exploded in rapid succession in Riyadh, a tense town north of Baghdad. At least five people, including three policemen, were killed. One of the bombs targeted a U.S. convoy but there was no report of casualties, police said.

Four American soldiers were injured in a car bombing today in Tikrit, the U.S. command said. Another car bomb targeted a multinational forces convoy on the road to Baghdad's international airport, injuring four soldiers, the command said.