A US Marine accused in the 2005 killings of 24 Iraqi civilians pleaded guilty Monday to negligence, as manslaughter charges were dropped in a deal under which he faces three months' imprisonment at most.

Staff Sergeant Frank Wuterich, the commander of a group of Marines whose other members have been exonerated, had faced nine counts of voluntary manslaughter and other charges over the killings in the Iraqi town of Haditha.

A sentencing hearing will be held Tuesday, said a spokesman for Camp Pendleton, south of Los Angeles, where the 31-year-old went on trial earlier this month.

Wuterich could also be reduced in rank to private, and have his pay cut by two-thirds for three months, he said, while stressing that the Marine accepted responsibility for giving orders which led to the deaths.

"Staff Sergeant Wuterich accepted responsibility... and agreed and admits that he gave a verbal order to shoot first, ask questions later, or don't hesitate to shoot, and words to that effect." said spokesman Joe Koppel.

"That verbal instruction caused his Marines to (not) positively identify targets in the two homes. And now, at the sentencing phase, he'll be held accountable for those actions."

In all, 24 Iraqi civilians were killed -- 19 in several houses along with the five men who pulled up in a car in the Iraqi town of Haditha on November 19, 2005.

The victims included 10 women or children killed at point-blank range. Six people were killed in one house, most shot in the head, including women and children huddled in a bedroom.

The other seven Marines charged in the case have been exonerated through various legal rulings, fueling anger in Iraq, where authorities had pushed for US troops to be subject to Iraqi justice before the US pullout in December.

Monday's plea deal was announced at the start of what would have been the third full week of the court-martial at Camp Pendleton, where Wuterich has been an active serving Marine pending the long-delayed legal action against him.

Wuterich "appeared before the military judge here today and pleaded guilty to one count of negligent dereliction of duty for his involvement in the death of Iraqi civilians in Haditha," said the statement.

"The defense and trial counsel initiated a discussion to resolve the case" under which Wuterich "agreed and admitted that he was derelict in the performance of his duty," it added.

A military judge will now determine the sentencing, which will then go to Lieutenant General Thomas Waldhauser, commander of the Marine Corps Forces Central Command, for final adjudication.

The Camp Pendleton spokesman added: "The max that he could face... is three months confinement, two-thirds forfeiture of pay for three months and reduction rate to E-1," or the rank of private, he said.

Asked about the other charges that Wuterich faced, he said: "Part of the guilty plea is that those will all be dropped."