More than 30 US Marines were killed today when their troop-transporting helicopter crashed in the desert of Western Iraq. It was the single most deadly incident for American forces since the invasion of Iraq almost two years ago.

The helicopter, a CH-53 Sea Stallion which can carry up to 55 passengers, was transporting troops to Western Iraq when it came down in the desert near the Jordanian border. Officials said more than 30 Marines aboard the craft had died; CNN put the exact toll at 31.

It was not clear whether the helicopter had been brought down by insurgents or high desert winds. But while military experts investigated the cause of the crash, five other American soldiers were killed in insurgent attacks across Iraq. At least ten Iraqis were also killed in a series of gun attacks, kidnappings and a triple car-bombing .

The deadliest day ever for US forces in Iraq came as Islamic and nationalist insurgents stepped up attacks to derail Sunday's nationwide elections, due to elect a national assembly that will decide on Iraq's long-term constitution.

At a White House news conference, President Bush said he had no details of the crash, which was still being investigated by the Defense Department.

"Listen, the story today is going to be very discouraging to the American people. I understand that," he said. "We value life and we weep and mourn when soldiers lose their life. But it is the long-term objective that is vital, and that is to spread freedom."

And Mr Bush said the violence would not stop the elections going ahead: "I urge all people to vote. I urge people to defy these terrorists. They have no clear view of a better future. They're afraid of a free society."

The Sea King helicopter went down about 1.20am near the town of Rutbah, about 220 miles west of Baghdad and near the Jordanian border, while conducting security operations, the military said in a statement. The aircraft was transporting personnel from the 1st Marine Division.

The military said a search and rescue team had reached the site and an investigation into what caused the crash was underway.

The United States has lost at least 33 helicopters since the March 2003 start of the Iraq conflict, according to a study by the Brookings Institution. At least 20 of them were brought down by hostile fire.

The previous worst incident took place on November 15, 2003, when two Black Hawk helicopters crashed in Mosul after colliding while trying to avoid ground fire, killing 17 US soldiers and wounding five. America's costliest day in Iraq - until today - was March 23, 2003, the third day of the war, when 28 U.S. soldiers died in various engagements, mostly in southern Iraq.

The crash pushed the US death toll in Iraq to past 1,400.

The mounting violence in Iraq - and the number of bodybags being sent back to the United States - has added to the pressure on the Bush adminstration to make clear its exit strategy from the conflict.

But Mr Bush sidestepped questions as to when the 150,000 American troops would start to pull out. "Obviously we’re going to have a troop level necessary to complete the mission, and that mission is to enable Iraq to defend itself from terrorists - homegrown or terrorists that come in from outside the country," he said.

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