"This morning at 4:44 am in Iraq, a coalition force team of eight soldiers -- seven Americans and an Iraqi army interpreter -- were attacked 20km west of Mahmudiyah," said spokesman Major General William Caldwell.

"As a result of this attack, five soldiers were killed in action and three are currently missing," he said, without clarifying whether the dead were all US soldiers or whether the Iraqi interpreter was among those killed.

"At the time of the attack, a nearby unit heard explosions and attempted to establish communications, but without success," he said in a statement.

"Coalition forces arrived within an hour, secured the site, and immediately initiated a search. The names of the soldiers are being withheld pending final identification and notification of next of kin," he said.

"Make no mistake -- we will never stop looking for our soldiers until their status is definitively determined, and we continue to pray for their safe return," he added.

Mahmudiyah lies 30km south of Baghdad in an area known for a strong insurgent presence and is nicknamed the "triangle of death."

Earlier, US marines on patrol on a road west of Baghdad near the restive city of Fallujah told an AFP reporter travelling with them that they had been told that the missing soldiers were thought to have been captured.

"We got word about 0900 (0500 GMT) that three soldiers were missing. Last we heard was that they were headed in this direction, presumed captured," said Gunnery Sergeant James Curtis of the US Marines 2nd Battalion 6th Regiment.

Curtis's patrol threw up a checkpoint on a stretch of road on the eastern edge of Fallujah in a bid to intercept the abductors if they attempted to take their captives there from western Baghdad.

Some of the trucks and cars arriving at the roadblock were marked as having been searched, showing that other military checkpoints had been set up on the road, which connects the western Baghdad suburb of Abu Ghraib to Fallujah.

While US forces in Baghdad and west and central Iraq come under attack daily from roadside bombs, snipers and guerrilla fighters, it is rare for insurgents to succeed in capturing American personnel.

Nevertheless, al-Qaeda in Iraq -- a Sunni group that has posted Internet videos of slain American and Iraqi hostages in the past -- has made it clear that capturing US service members is a priority.

An alleged senior al-Qaeda in Iraq militant said during an interview on a Jihadi Internet chatroom last month that the organisation was seeking to capture and kill more American soldiers.

"The beheadings are still happening, but we have an order not to broadcast them," said Abu Adam al-Maqdisi, according to terrorism consultant Evan Kohlmann, who posted a translation of the remarks on his own website.

Shia militants have also captured Americans in the past.

On January 20, gunmen disguised in US army uniforms breezed past security checks and attacked a provincial security building in central Iraq's Karbala province during a visit by American troops to their Iraqi counterparts.

One US soldier died in the assault, and four more were captured. Their slain bodies were later found with the attackers' abandoned SUV vehicles, along with disguises and US-style weapons and radios.

The US commander in Iraq, General David Petraeus, later said the cell suspected of having carried out the murders had received support and funding from Iran, but did not link Tehran directly to the attack.

In June 2006 a posting on an al-Qaeda-linked website said the group's alleged leader -- known under the nom de guerre "Sheikh Abu Hamza al-Muhajer" -- had ordered the killing of two kidnapped American soldiers.

The two US soldiers were later found south of Baghdad, their bodies showing signs of brutal torture, according to the Iraqi defence ministry.

US Army Sergeant Ahmed Qusai al-Taie, an Iraqi-born American soldier, went missing on October 23 last year after he left the heavily guarded Green Zone without permission in order to visit his wife at a family home in Baghdad.

He has never been found and is believed to be held by a Shia gang.