Soldiers combed the crash site for survivors

It is the highest number of casualties suffered by the US-led coalition in a single incident since Saddam Hussein was toppled in April.

The helicopter came down in a cornfield near the flashpoint town of Falluja, 50 kilometres (32 miles) west of the capital.

Later the US Army said two US civilian contractors working for it in Falluja were killed by a roadside bomb.

The improvised device exploded as their truck was passing by. Another civilian was injured in the blast, a spokesman said.

Iraqi witnesses said the helicopter was hit by one of two surface-to-air missiles fired at it, but the US military has not confirmed the cause.

One military spokesman said the helicopter was hit by an "unknown weapon", but later, the military said it might have crashed while taking evasive action.

"We are aware of eyewitnesses seeing what they presumed to be missile trails," US army Colonel William Darley told reporters.

'Tragic day'

The helicopter was one of two twin-rotor Chinooks flying nearly 60 personnel from a US military base to Baghdad International Airport, from where they were due to fly abroad for rest and recreation.

Falluja's anger

Sunday marked the start of an expanded leave programme for US personnel - many of whom have been in the region for more than a year.

The helicopter was almost totally destroyed in the incident, which happened at about 0900 (0600 GMT). Television pictures showed US personnel recovering the dead and wounded from smoking debris.

Responding to the news, US Defence Secretary Donald Rumsfeld said the US would continue its war on global terror and efforts to stabilise Iraq.

"Clearly it is a tragic day for Americans," Mr Rumsfeld told ABC television.

"In a long hard war we are going to have tragic days."

Some Iraqis in Falluja expressed delight at Sunday's attack.

"The Americans are pigs. We will hold a celebration because this helicopter went down - a big celebration," an Iraqi farmer near the crash site told Reuters news agency.

The downing of the helicopter was one of a number of attacks on US forces in Iraq on Sunday.

One American soldier died when his convoy was attacked in the early hours in Baghdad.

Fact file: Chinook

Falluja lies within the so-called "Sunni triangle" of central Iraq - a largely Sunni Muslim area where resistance to the US-led coalition's occupation has been intense.

US military officials have repeatedly warned that hundreds of surface-to-air missiles remain unaccounted for in Iraq.

The Chinook, which has a crew of four, is a heavy-lift helicopter used primarily for moving troops and transporting artillery.

Intensified attacks

A total of 138 American troops have now been killed in attacks since US President George W Bush declared major combat operations over on 1 May - more than died in the war itself.

Attacks on coalition troops have intensified in the past week, reaching an average of more than 30 a day.

MAJOR POST-WAR ATTACKS 27 Oct: 36 killed in co-ordinated suicide attacks on Red Cross HQ and police stations in Baghdad 29 Aug: Shia Muslim cleric Ayatollah Mohammed Baqr al-Hakim among 80 killed in bombing in Najaf 19 Aug: UN special representative among 22 killed in attack on UN HQ in Baghdad

The BBC's Jill McGivering in Baghdad says the planning and execution of some recent attacks has led to speculation the militants opposed to the coalition have now formed into a more cohesive, better organised force.

There had been rumours that this weekend would see co-ordinated attacks on coalition targets.

A senior British member of the Coalition Provisional Authority in Iraq, Sir Jeremy Greenstock, told the BBC that the people carrying out the attacks were a "nasty mix".

He said they were supporters of former Iraqi leader Saddam Hussein, "imported terrorists", and criminals released from jail by the former regime before it fell.