The casualties occurred in "a large explosion," a US central command spokesman said. He said he had no idea what caused the blast.

The spokesman said three soldiers were killed and one solider was injured. Speaking on condition of anonymity, the spokesman said he had no other details.

Witnesses, however, spoke of two roadside bombs in Khaldiyah, 50 miles west of Baghdad, and close to Fallujah town. The area is a hotspot of guerrilla activity where several attacks have taken place against US-led coalition forces.

The witnesses said the first one exploded next to a passing US military convoy followed by a second blast when reinforcements arrived.

"One of our units was ambushed near Fallujah ... involving two coalition vehicles," the military spokesman said earlier.

Abdul Hamid Marzouq, a nurse at a nearby hospital where the casualties were taken, said two Iraqis were killed -- Hadi Abd Shehab, the director of agriculture of Khaldiyah, and Hamd Nayef, a taxi driver.

Nayef, who was driving by at the time of the explosion, was injured in the head and face, said Marzouq. He said three people were injured.

Marzouq said Shehab died of a gunshot wound to the stomach. Witnesses said he was shot while standing in his office close the blast scene, and died on way to the hospital.

It was not clear who shot him.

Nameer Mohammed, who said he was standing about 500 yards away when the attack occurred, claimed American soldiers fired randomly after the blasts. That could not be independently confirmed.

Mohammed described seeing a US military vehicle on fire after the first blast.

As more American forces came to the scene, another bomb went off, setting fire to a second vehicle, he said.

Mohammed said he carried several injured people to the hospital.

The last serious attack in Khaldiyah took place on Saturday when a car bomb killed three American soldiers at a checkpoint and injured six in addition to several Iraqi civilians. Two other American soldiers were killed in Fallujah the same day.

Khaldiyah and Fallujah are part of the Sunni Triangle, the area in central Iraq where most of the anti-US attacks by die-hard Saddam Hussein loyalists have taken place.

The US military says the attacks have reduced in number since Saddam's arrest on December 13.