George Bush said today that he was troubled by allegations that US Marines massacred unarmed Iraqi civilians and that, “If in fact laws were broken, there will be punishment”.

It was the President’s first public comment on allegations that Marines killed 24 Iraqis in the town of Haditha last November.

The White House has already promised full public disclosure of the results of a military probe into the alleged massacre, after renewed accusations of a cover up by the Marine Corps.

The alleged killing spree took place on November 19 last year, prompted, it is claimed, by the death of a Marine driver blown up when his Humvee was hit by a roadside bomb.

Two inquiries are under way into the incident and supposed cover-up. Leaked photographs taken by another Marine unit and a video by a trainee Iraqi journalist suggest that many of the victims were shot, execution-style, in the head and chest in their houses. Five men were killed in a taxi at a checkpoint.

The Marines had initially reported the deaths as another bloody incident in Iraq, saying that 15 civilians and one Marine had been killed by a roadside bomb in the town, which is a hotbed of the insurgency.

Tony Snow, the White House spokesman, said last night that the US Marine Corps was taking an "active and aggressive role" in investigating the allegations.

"I have been told and was assured earlier today when I called about it that when this comes out, all the details will be made available to the public," he added.

Mr Snow said that President Bush had only heard about the incident at al-Haditha after a Time magazine report in March and was waiting for the military to complete its investigation.

Neverthless, the Bush Administration faces damaging accusations of a cover-up despite the investigations. John Murtha, a Democratic Representative and critic of the Iraq war, said that the killings would rival the Abu Ghraib scandal.

Twelve Marines are under investigation, some of whom could face murder charges in what would be the worst case of criminal misconduct by US troops since the invasion three years ago. Three Marine officers have since been relieved of their command.

Mr Murtha, himself a former Marine, charged that US military authorities had paid compensation to the families of the victims, indicating they had assumed responsibility for the deaths. "They paid people $1,500 to $2,500. This doesn’t happen unless it comes at the highest authority," Mr Murtha told CNN.

Asked if he meant victims’ compensation, Mr Murtha said: "Yes. And that doesn’t happen ... if it’s an explosive device."

Mr Murtha repeated his accusation that the Marines had sought to cover up the killings."This is what worries me. We’re fighting a war about America’s ideals and democracy’s ideas and something like this happens, they try to cover it up," he said. "It is as bad as Abu Ghraib, if not worse."

He added: "They knew the day after this happened that it was not as they portrayed it. They knew that they went into the rooms, they killed the people in the taxi. There was no firing at all. And this comes from the highest authority in the Marine Corps, so there’s no question in my mind."

The revelations about the al-Haditha killings have dealt another blow to America's standing in Iraq. Nouri al-Maliki, the new Iraqi Prime Minister, said yesterday that there was a "limit to the acceptable excuses".

There was more embarrassment for the Bush Administration after the new Iraqi ambassador to the United States accused the Marines of "intentionally" killing his cousin al-Haditha in a separate incident five months earlier.

Speaking only hours after presenting his credentials to Mr Bush at the White House, Ambassador Samir al-Sumaidaie told CNN how Mohammed al-Sumaidaie, a 21-year-old engineering student, was killed after opening the door of the family house to US Marines on June 25 last year.

"I believe he was killed intentionally. I believe he was killed unnecessarily," Mr al-Sumaidaie said. He said that a military investigation had found that there was no unlawful killing but added: "I would like further investigation."

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