A third member of an alleged rogue US army unit in Afghanistan pleaded guilty in a plea deal over shock killings of local civilians that embarrassed the US army.

This file photo, released by the US Army, shows US Private Andrew Holmes.

Private First Class Andrew Holmes was accused of being a member of a five-person "kill team" implicated in the slaying of three Afghans while stationed in Afghanistan's southern Kandahar province in January 2010.

After negotiations, Holmes on Thursday pleaded guilty to murder, but not to premeditated murder, during a court-martial hearing at Joint Base Lewis-McChord near Tacoma, in the western state of Washington. He was expected to be sentenced on Friday.

Speaking in a clear voice, the 21-year-old from Boise, Idaho also pleaded guilty to two other offenses: possessing a finger harvested from an Afghan corpse and smoking marijuana in hashish form while deployed.

As part of the agreement, prosecutors dismissed three charges, including the two major charges of premeditated murder and conspiracy.

After entering his pleas, Holmes read a prepared statement explaining his actions on January 15, 2010 and answered questions about the incident from the presiding judge Kwasi Hawks.

According to Holmes, the incident began when fellow soldier Jeremy Morlock motioned for an Afghan boy to come closer.

When the boy was about 15 feet (five meters) away, Morlock threw a grenade at him. Holmes then fired his squad automatic weapon at the boy at Morlock's urging.

"I fired six to eight rounds -- and have regretted it ever since," Holmes told the judge.

The squad had also discussed scenarios to make the killing of Afghan civilians appear justified.

Holmes voiced regret for the killing, admitting it was "reckless" but saying it was not his intention to murder the civilian. Quizzed by the judge, he said he believed the bullets from his gun killed the Afghan boy, not the grenade.

The soldier also recalled his doubts about the Afghan civilian being a threat and called himself "naive" in his assessment of the situation at the time. He was 19 at the time of the shooting.

"He pleaded guilty to doing something reckless that killed a human being," defense attorney Daniel Conway said after the proceedings.

Two other members of the alleged rogue unit, Morlock and Adam Winfield, reached plea deals earlier this year as the US Army works to conclude the high-profile war crimes case.

Morlock received a 24-year sentence in March. In August, Winfield received a three-year sentence after pleading guilty to involuntary manslaughter, saying he was threatened by his platoon sergeant to take part in the killings.

Holmes, along with Morlock, was one of the US soldiers shown posing for the camera with an Afghan corpse, in shocking photos published by German news magazine Der Spiegel.

Two other soldiers -- Michael Wagnon and alleged ringleader Staff Sergeant Calvin Gibbs -- still face charges for their alleged role in the killings.

The soldiers were members of Bravo Company, 2nd Battalion, 1st Division's Stryker brigade, based out of Joint Base Lewis-McChord south of Seattle.