The cause of the crash was not immediately known.

"Anytime we lose lives, it is a sad moment," President George W. Bush told reporters at the White House.

Lieutenant General John Sattler, the commander of marine forces in Iraq, said 30 marines and a sailor were killed when a CH-53E Super Stallion helicopter crashed near Ar Rutbah in western Iraq.

"The 1st Marine Expeditionary Force mourn the tragic loss of our brothers in arms," Sattler said in a statement from Camp Fallujah, the marine headquarters in western Iraq.

"A recovery team is at the crash site and the cause of the crash is currently under investigation," he said.

The CH-53 helicopter was carrying troops from the 1st Marine Division on operations near Ar Rutbah in western Iraq when it crashed at 1:20 am local time, the Marine Corps said in a statement.

A marine official, speaking on condition of anonymity, told AFP that all 31 people aboard the helicopter were killed. They were from the 1 Marine Division and the 3rd Marine Aircraft Wing.

General John Abizaid, the commander of US forces in the region, also told reporters there were no survivors.

"Weather was bad. We don't know of any enemy action. The investigation will have to continue. There will have to be more that comes out of this to learn what happened," he said after meeting with members of Congress.

Abizaid said the marines were on "a routine mission in support of the elections."

A marine official said hostile fire had not been ruled out as a cause but there were no immediate indications as to the cause.

Four other marines were reported killed in action Wednesday in western al-Anbar province and a soldier was killed in an attack on a patrol north Baghdad, raising the US death toll for the day to 36, the highest single day US toll of the war.

It was also the deadliest helicopter mishap so far with casualties far exceeding the 17 US military personnel killed when two UH-60 Blackhawk helicopters collided near Mosul November 13, 2003.

On November 2, 2003, 16 soldiers were killed when a shoulder-fired surface-to-air missile brought down army CH-46 Chinook helicopter.

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