BAGHDAD, November 22, 2004 (AFP) - "You are killers, not murderers. You are warriors not war criminals. Don't cross that line."

Those were the words of a US officer to his men before they took part in the recent assault on the rebel bastion of Fallujah.

Just days later, one US marine was in the spotlight, with questions being asked around the world about whether he was a murderer and a war criminal.

His shooting of a wounded Iraqi, caught on tape and beamed around the world ever since, has raised questions about the degree of military restraint and has fanned Arab resentment.

After Iraqi Prime Minister Iyad Allawi, the Fallujah campaign's staunchest supporter, voiced his concern over the incident, the International Committee of the Red Cross sharply criticised the "utter contempt" for humanity shown by both sides in the conflict.

The shooting occurred on November 13 during the search of a mosque. A trooper raised his rifle and shot point blank at an apparently unarmed, wounded Iraqi who was slumped against one of the walls.

The marines said the rifleman was withdrawn from combat pending the results of an investigation, but the graphic footage had already made tempers boil in the region, months after the scandal over US troops' abuse of inmates at the Abu Ghraib prison.

But reports from newsmen embedded with the US troops during the assault launched on November 8 suggest that the shooting may not have been an isolated incident.

Instead, it may have simply been the only one caught on tape, an illustration of the looser rules of engagement authorised for the Fallujah offensive.

"The enemy can dress as a woman, the enemy can be faking to be dead," said one company commander to his marines before entering the heart city. "So shoot everything that moves and everything that doesn't move," he said.

The photographer embedded with this unit, which carried out some of the most dangerous missions on the frontlines of the Fallujah battle, said the rules of engagement were gradually modified as the situation evolved.

"A marine was killed when a unit entered a house. They pulled out and dynamited the building, but when they moved back in, an arm stuck out from under the rubble and threw a grenade," he said.

The photographer, who did not wish to identify his unit, said the fear of suicide bombers combined with the discovery that fighters were taking amphetamines and adrenaline prompted his platoon to take new measures.

"From that point on, the rule was the so-called 'double tap': two bullets in every body," he said.

The night before the assault began, the order came down that troops could shoot any male on the street between the ages of 15-50 if they were viewed as a security threat, regardless of whether they had a weapon.

When marines asked a gunnery sergeant for clarification, he told his men if they saw any military aged males on the street "Drop 'em."

The marines had issued special rules of engagement, particularly for the Fallujah operation, that emphasised anyone considered a danger could be shot.

After one marine was killed and five were wounded on the second day of the assault, the military command ordered platoons to spray homes with machine-gun and tank fire before entering them, in an effort to kill insurgents lurking inside waiting for them.

The US military said on November 14 that more than 1,200 rebels had been killed since the start of the operation, the largest toll in Iraq since the 2003 invasion. It said Thursday that 51 marines and eight Iraqi soldiers were killed in the clashes.

However, there no casualty toll for civilians has been made known.

Although the vast majority of Fallujah's residents fled the city before the onslaught, some families decided to stay behind, and many are feared to have been killed in the violence.

The Iraqi Red Crescent Society said Thursday that some 150 families were still trapped inside Fallujah, as the US-led operation was winding up, with troops still fighting the last pockets of insurgents.

Marine Lieutenant Colonel Leonard DiFrancisci put the number of civilians left in Fallujah at under 1,000.