AMERICAN forces yesterday suffered their worst day of losses in Iraq in nearly two years when a US Marine helicopter crashed, killing thirty-one people, and five other troops were killed by insurgents.

With only four days to go before Iraq holds elections, the death toll dealt a heavy blow to American efforts to establish security over the country and encourage the participation of as many as possible.

Election workers and local politicians bore the brunt of the latest violence. In Arbil, northern Iraq, a truck bomb exploded outside the headquarters of a Kurdish party, killing 15 people and injuring 80 others.

In Mosul, another northern city, insurgents released a video showing three Iraqi electoral officials who had been taken hostage at gunpoint.

The film showed a hooded militant armed with a pistol reading out a statement, as another masked gunman crouched with a rocket-propelled grenade launcher on his shoulder. “We are Mujahidin in the province of Nineveh. What they call elections have no basis in the Islamic religion and that’s why we will hit all election centres,” the statement said.

The US military did not say what caused the giant US Marine Chinook transport helicopter to crash in bleak desert terrain near the town of Rutbah on the Jordanian border during the early hours.

The twin-rotor helicopter may have been brought down by ground fire, like more than two thirds of the 33 other US helicopters lost in Iraq. American helicopters, which often travel at night to avoid being hit, frequently come under fire. Rutbah is known as one of the more active insurgent areas in Anbar province, the main militant stronghold.

But US officials said it was more likely that sand storms caused the crash. “The weather was bad,” General John Abizaid, chief of US Central Command, said. “We don’t know of any enemy action; the investigation continues.”

President Bush conceded that the news would be “very discouraging” to the American public, a majority of whom now believe the war to be a mistake, according to recent polls.

“I understand that,” Mr Bush told a hastily arranged news conference yesterday. He tried to focus attention on US goals in Iraq and the wider Middle East and away from details of the disaster. “It is the long-term objective that is vital — and that is to spread freedom,” he said.

Yesterday’s deaths brought the toll for US troops in Iraq to more than 1,400. The previous single deadliest incident was another helicopter crash in November 2003 when 17 soldiers were killed after two army Black Hawk helicopters collided trying to avoid ground fire. To add to the US military’s woes, four more Marines were killed elsewhere in Anbar province yesterday and an American soldier died when he was hit in a rocket attack near Baghdad. Four American soldiers were also injured by a car bomb near Saddam Hussein’s home town of Tikrit. Another car bomb targeted an American convoy on the road to Baghdad international airport, injuring four soldiers.

General George Casey, the commander of US forces in Iraq, gave warning that the violence could intensify up to and during the elections on Sunday.

Mr Bush set the bar low for a successful election, declining to endorse any turnout figure below which the result would lack credibility. “The fact that they’re voting in itself is successful,” he said.

He conceded that some Iraqis were intimidated by terrorist threats. But he said that “millions” would show their bravery and love of freedom by voting and that he anticipated a “grand moment in Iraqi history”.

He said: “I urge all people to vote. I urge people to defy these terrorists. These are exciting times for the Iraqi people.”

General Casey said that American troops will have to remain in Iraq for the foreseeable future because Iraqi security forces were not capable of fighting the growing insurgency on their own.

“Can I sit here and look you in the eye and say that the Iraqi security forces, guaranteed 100 per cent, are going to be able to defeat this insurgency by themselves? Of course not,” General Casey said.

Times Recommends

• The Japanese Prime Minister is doomed: or is he?
• Brighton company 'involved in cyber attack'
• The art of survival: it’s all in the mind