The allegations in Newsweek magazine contribute to an ever more disturbing portrait of embattled marines under high stress, some on their third tour of duty after ferocious door-to-door fighting in the Sunni insurgent strongholds of Falluja and Haditha.

The wife of the unnamed staff sergeant claimed there had been a "total breakdown" in the unit's discipline after it was pulled out of Falluja in early 2005.

"There were problems in Kilo company with drugs, alcohol, hazing [violent initiation games], you name it," she said. "I think it's more than possible that these guys were totally tweaked out on speed or something when they shot those civilians in Haditha."

The troops in Iraq have been ordered to take refresher courses on battlefield ethics, but a growing body of evidence from Haditha suggests the strain of repeated deployments in Iraq is beginning to unravel the cohesion and discipline of the combat troops.

"We are in trouble in Iraq," Barry McCaffrey, a retired army general who played a leading role in the Iraq war, told Time magazine. "Our forces can't sustain this pace, and I'm afraid the American people are walking away from this war."

The Newsweek account described a gung-ho battalion that had staged a chariot race, complete with captured horses, togas and heavy metal music, before the battle for Falluja in late 2004. The marines were given loose rules of engagement in the vicious urban warfare that followed.

"If you see someone with a cellphone," said one of the commanders was quoted as saying, half-jokingly, "put a bullet in their fucking head".

At one point in the battle, a marine from the 3rd battalion was caught on camera shooting a wounded, unarmed man as he lay on the ground. However, the marine involved was later exonerated.

The third battalion lost 17 men in 10 days in Falluja and by the time the troops arrived in Haditha, in autumn last year, it was clear morale had plummeted. A Daily Telegraph reporter who visited its headquarters early this year at Haditha Dam, on the outskirts of the town, described it as a "feral place" where discipline was "approaching breakdown". He reported that some marines had left the official living quarters and had set up separate encampments with signs ordering outsiders to keep out.

Other observers, however, have come away from time spent with the marines with different impressions. Lucian Read, a photographer who spent five months with Kilo company, said it was generally well led, although sometimes squads had to go on patrol without an officer because there were not enough to go around.

Mr Read told Time magazine that Kilo company was the "most human" of the many units he had accompanied in Iraq. "They were never abusive," he said. "There was a certain amount of antagonism and frustration when people didn't cooperate. But it's not like they had 'kill 'em all' spray-painted on the walls."

Three senior officers in the Haditha-based 3rd battalion of the first marine regiment, known as the Thundering Third, have been relieved of duty because of a "lack of confidence" in their leadership.

The officers include Captain Lucas McConnell, the head of Kilo company, which was directly involved in the deaths of 24 unarmed Iraqis there on November 19.

Another captain from the battalion, James Kimber, was relieved of duty for a separate incident, according to his lawyer, who said his subordinates in India company had sworn and derided Iraqi security forces in an interview with Sky News.

The commander of the third battalion, Lieutenant Colonel Jeffrey Chessani, has also been made to step down pending the outcome of the Haditha investigation.

A criminal investigation conducted by navy investigators into the Haditha killings is still under way, but a parallel army inquiry into the wider issues has been completed. However, a military official said some findings might be withheld pending the principle inquiry findings.

On Saturday the Iraqi government rejected the findings of a US inquiry into the death of nine civilians in a US raid in the town of Ishaqi and said it would conduct its own investigation.