A US military spokesman said 16 soldiers were killed and more than 20 wounded when an "unknown weapon" struck a Chinook helicopter early yesterday morning, sending it spinning from the sky to crash in a field near the village of Hasi, six miles south of Falluja, a stronghold of anti-American militancy.

In another incident in the town, two American civilians working under contract to the US Army Corps of Engineers were killed when their convoy hit a roadside bomb, a military spokesman said.

Even before the attacks the American public's confidence in the administration's ability to stem the growing number of casualties and the escalating cost of the war had fallen to its lowest level, according to a poll released yesterday.

The Washington Post-ABC survey shows that the number of Americans who disapprove of Mr Bush's handling of Iraq has climbed to 51%, the highest level yet, while those who oppose the president's appeal for $87bn (£51bn) to finance the occupation have risen to 64%.

As the attacks on US soldiers increased to an average of 33 a day, the administration expressed regret for the deaths but vowed to pursue its present course.

Mr Bush was at his ranch in Texas but a spokesman travelling with the presidential group said: "Our will and resolve are unshakable."

The defence secretary, Donald Rumsfeld, said: "In a long, hard war we are going to have tragic days. But they are necessary. They are part of a war that is difficult and complicated."

Witnesses said they saw two missiles fired at the helicopter, which was one of two carrying more than 50 soldiers to Baghdad airport, reportedly to fly home on leave.

The attack ended a week of growing carnage that has exposed the deteriorating security situation and the increased vulnerability of US troops to resistance fighters.

Last Sunday a rocket attack on a hotel housing hundreds of coalition staff killed one and injured 15. A day later four coordinated suicide bombings in Baghdad killed 36 people and wounded more than 200.

Mohammed al-Qubaisi, a farmer, said he ran out of his house, half a mile away, when he heard the whoosh of missiles fired at the helicopter.

"They fired from a date grove. I heard the copter's engines getting louder, and then a big bang. I wanted to help, but couldn't get near because of American soldiers."

The second helicopter hovered over the stricken Chinook for some minutes and then set down, apparently to try to help extinguish a fire.

The incident was the third time an American helicopter had been shot down in six months. A Blackhawk crash-landed on October 25 in Tikrit after being hit by an unknown weapon, injuring a crew member. On June 12 an Apache attack helicopter was shot down in the western desert: two crew were rescued.

American officials said recently that hundreds of shoulder-fired surface-to-air missiles had gone missing in Iraq since the collapse of Sad dam Hussein's regime in April.

There have been many reports of such missiles being fired at aircraft as they approach Baghdad airport, but until yesterday none had struck to such devastating effect.

In Falluja a crowd of young men danced in the streets shouting anti-American and Islamic slogans at the news of the attacks on the Chinook and the contractors, who were engaged in destroying Iraqi munitions, which have been used to make roadside bombs.

"A roadside bomb struck a car and the four people inside were burned alive," said Ammar Majid, who works at a local car dealer's. "We were all very happy today."

In Baghdad a soldier of the 1st Armoured Division was killed just after midnight when a bomb exploded as his vehicle passed by.