The US military has announced it is looking into whether an American marine in Falluja shot dead a severely wounded Iraqi insurgent at point-blank range.
The video showed a wounded insurgent on the floor of the room
Television footage shows US soldiers entering a building as injured prisoners lie on the floor.
The soldier, who has not been identified, has been removed from the field and faces possible charges.
Lt Gen John Sattler promised to pursue the facts of the case "thoroughly" before taking further measures.
The BBC's James Robbins says the incident could prove highly damaging and that the US military will need to answer key questions about whether the rules of engagement were broken during the incident.
It must explain, he says, whether wounded combatants were abandoned, or killed, illegally.
But the BBC's Baghdad correspondent Caroline Hawley says most Iraqis will not be surprised, after the scandal of US abuse of Iraqi prisoners at the Abu Ghraib facility in Baghdad.
US-led forces said they have now gained overall control in Falluja, trapping rebels in the south of the city.
The images of the alleged point-blank shooting of an Iraqi insurgent were taken by an NBC reporter embedded with the US troops in the Sunni city under assault.
They show a group of marines from the 3rd Battalion, 1st Regiment, armed with rifles, entering a building near a mosque last Saturday, 13 November.
The mosque had been used by insurgents to attack US forces, who had stormed it a day earlier, killing 10 militants and wounding five.
At least three severely wounded men are seen in a room inside the building - two are slumped against one of the walls, partially covered with a blanket.
The NBC's Kevin Sites says the wounded men had been left in the mosque after being treated by a group of marines following Friday's fighting.
Mr Sites says soldiers from a different unit went and apparently shot the men again on Saturday without knowing whether they were armed.
"Then one of the marines points his rifle at the head of one of the injured, an old man, saying, 'He's faking he's dead'," Mr Sites' description continues.
"The sound of a shot is then heard. And in the background, another soldier says, 'Well, he's dead now'."
The day before the incident, the marine who allegedly did the point-blank shooting had reportedly been shot in the cheek and returned to duty, while another marine in the same unit had been killed by explosives planted in the body of a dead insurgent.
Gen Sattler, commander of the 1st Marine Expeditionary Force, said the purpose of the investigation was to determine "whether the marine acted in self-defence, violated military law or failed to comply with the law of armed conflict".
"We follow the law of armed conflict and we hold ourselves to a high standard of accountability," Gen Sattler said.
UK Defence Secretary Geoff Hoon said he was determined "as the Americans are, that there should be a thorough and proper investigation".
"If the film shows what it appears to show, then the Americans must look at it very hard indeed," Mr Hoon told the BBC.
US marines have been battling with sporadic but fierce pockets of resistance inside the city. They say they have already killed 1,200 insurgents in the week-long assault.
The offensive is now concentrated mainly in southern districts, where small groups of rebels are "fighting to the death", US commanders say.
Aid agencies are warning of a humanitarian disaster in Falluja, which has been without water or electricity for a week.