WASHINGTON, Jan 26 (AFP) - A Marine Corps transport helicopter crashed in Iraq early Wednesday, killing all 31 US marines aboard, military officials confirmed, marking the biggest single day loss of US troops since the 2003 invasion.

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.

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

A marine official, speaking on condition of anonymity, told AFP that all 31 marines aboard the helicopter were killed.

General John Abizaid, the commander of US forces in the region, 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 of the crash.

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.

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