A United States Marine has been withdrawn from duty in Fallujah after claims that an apparently injured and unarmed Iraqi prisoner was shot dead in a mosque in the city.

Graphic video footage was shown on American television last night of a US Marine appearing to shoot dead at point-blank range the severely wounded and unarmed man. First aired by NBC, but quickly broadcast by all the major networks, the soldier appeared to turn his weapon on the wounded militant as he lay on the mosque floor.

The Iraqi was one of five wounded prisoners left in the mosque after Marines had fought their way in on Friday and Saturday. The Pentagon said last night that an investigation was under way.

The pool report, by the NBC correspondent Kevin Sites, said that the mosque had been used by insurgents to attack American forces, who stormed it and an adjacent building, killing ten militants and wounding the five.

Mr Sites said that the wounded had been left in the mosque for others to pick up and move to the rear for treatment. No reason was given why that had not happened.

A second group of Marines entered the mosque on Saturday after reports that it had been reoccupied. Footage from the television crew showed the five insurgents still in the mosque, although several appeared to be close to death, Mr Sites said. He said that one Marine noticed one of the prisoners was still breathing.

A Marine can be heard on the footage saying: "He’s f****** faking he’s dead." The Marine then raises his rifle and appears to fire into the man’s head. Mr Sites said the shot man "did not appear armed or threatening in any way".

The First Marine Division is investigating the allegation of the unlawful use of force in the death of an enemy combatant in Fallujah during combat operations on Saturday, according to a military statement.

"We follow the law of armed conflict and hold ourselves to a high standard of accountability," said Lieutenant General John F. Sattler, the commanding general of the 1st Marine Expeditionary Force. "The facts of this case will be thoroughly pursued to make an informed decision and to protect the rights of all persons involved."

The investigation, which began as soon as allegations were brought forward, will determine whether the Marine acted in self-defence, violated military law or failed to comply with the law of armed conflict.

Elsewhere in Iraq, about 1,200 US troops began an offensive today to secure part of Mosul, Iraq's third largest city, from insurgents who have rampaged through the city over the past week.

A US military spokeswoman said the troops, helped by Iraqi security forces, began moving in to secure about a dozen police stations in the western part of the city, closing off Mosul's five bridges over the River Tigris.

"Offenisve operations have begun on the western side of the river to clear out final pockets of insurgent fighting," said Captain Angela Bowman.

"It's a significant operation to secure police stations in the area and make sure that they can be put to use again."

Residents said US warplanes swooped above the city and helicopters hovered overhead. Loud explosions and gunfire were heard near the American base on the northern edge of Mosul.

Witnesses said three police stations already under the control of insurgents was blown up this morning as the militants fled ahead of the offensive.

Groups of up to 50 militants stormed through Mosul last week after US and Iraqi forces began an offensive to recapture the rebel-held Sunni Muslim city of Fallujah. They attacked police stations, looting weapons, flak jackets and vehicles.

Nine of Mosul's 33 police stations were overrun and some briefly held by the insurgents, prompting the sacking of the city's police chief. US and Iraqi forces regained control of one of the police stations on Sunday in a gun battle lasting several hours.

The US has assigned a brigade of troops - around 5,000 men - and the Iraqi Government has committed another brigade to restoring order to Mosul, but only a small proportion of those troops were involved in today's offensive.

Insurgents today attacked the offices of a key Kurdish party in Mosul, the Patriotic Union of Kurdistan. A party official said three insurgents were killed in a bloody gun battle.

A mass insurgent uprising began in Mosul last week in apparent support of militants in the rebel bastion of Fallujah, just days after the start of the U.S. offensive there. Masked and armed bands of men stormed more than a half dozen police stations, bridges and political offices in the city, clashing with U.S. troops and Iraqi forces.

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