MCCHORD, Washington (AFP) - A US soldier has pleaded guilty to killing 16 Afghan villagers, in exchange for escaping execution over a chilling massacre he could not explain.

In a plea deal worked out by his lawyers, Sergeant Robert Bales on Wednesday admitted 16 counts of murder over last year's slayings in southern Afghanistan that strained already tense US-Afghan ties.

Military Judge Colonel Jeffery Nance accepted the pleas, and ruled that Bales will face a maximum of life behind bars without eligibility for parole.

Prosecutors had said they would seek the death penalty.

The soldier's lawyer John Browne said he hoped his client could be out of jail after 10 years. He will be sentenced in August.

Bales initially appeared a little choked up when responding to a judge's request before a packed courtroom at Joint Base Lewis-McChord south of Seattle in northwestern Washington state to recount the events of March 11, 2012.

"I formed the intent to kill and then did kill by shooting with a firearm and burning her," he said, repeating the phrase for each of the 16 murder counts against him.

Asked why he had killed the villagers, he said: "Sir, as far as why, I've asked that question a million times since then. There's not a good reason in this world for why I did the horrible things I did."

Speaking in a steady voice once he had answered a few questions, Bales recalled he had an M4 rifle and a 9mm pistol.

"I don't know which firearm produced which injuries, but I used both weapons," he said.

Asked whether he remembered burning the bodies, Bales said: "I remember there being a lantern in the room, I remember there being a fire after that situation... but to say I remember throwing the lantern on those people, I don't recall that."

Bales, who looked older than his 39 years, was flanked by his lawyers Browne and Emma Scanlan in the courtroom at the base, where he has been held pending court-martial proceedings.

Ms Scanlan entered guilty pleas for all charges against him, which also include six of attempted murder and seven of assault. Seventeen of the 22 victims were women or children and almost all were shot in the head.

Bales allegedly left his base in the Panjwayi district of southern Kandahar province to commit the killings. Nine of those killed were children.

He recalled how he came across a woman as he approached a compound. "I had a brief struggle with an older woman... upon completion of that struggle, I did form the intent to kill the people in that compound," he said.

Bales confirmed that he had tested positive for the anabolic steroid stanozolol, which he said had "increased my irritability and anger."

Mr Browne announced last week that he had reached an agreement with the military to "take the death penalty off the table" if Bales would plead guilty, and said the soldier was sorry.

Bales did not apologise during his testimony, before the judge accepted his pleas.

Speaking after the hearing finished, Mr Browne said Bales had not apologised in court "because it's not relevant today."

But he added: "He's very remorseful for everything," and suggested he could say sorry at the sentencing phase.

Asked if Bales could get life with the possibility of parole, and be freed in 10 years' time, he said: "Yes, that could be a possibility."

"There's no death penalty on the table and there's an opportunity for life with parole," Mr Browne added.

A sentencing trial set for August 19 will be decided by a 12-member jury, a third of which will be enlisted officers.