A US soldier pleaded guilty Wednesday to killing 16 Afghan villagers last year, as he faced a military hearing hoping to escape a death sentence.

Sergeant Robert Bales, 39, pleaded guilty to 16 counts of murder over the massacre in southern Afghanistan in March last year. His lawyer said last week he would admit guilt in exchange for prosecutors not seeking his execution.

Bales was flanked by his lawyers John Browne and Emma Scanlan in a packed courtroom at Joint Base Lewis-McChord south of Seattle in Washington state, where he has been held pending court martial proceedings.

Scanlan entered guilty pleas for all charges against him including 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 pleaded not guilty to one count of wrongful endeavor to impede an investigation by damaging a laptop.

When asked by military judge Colonel Jeffery Nance if he understood that a guilty plea was final, Bales responded crisply, "Yes, sir."

A date of 19 August was set for a sentencing trial. Bales requested trial by 12-member jury including one-third enlisted officers. Prosecutors did not immediately confirm that they were no longer seeking the death sentence.

In death penalty cases, a unanimous verdict is required.

Bales' lawyer John 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.

Asked if Bales was sorry, Browne said: "Absolutely. And I think that will become clear as the process goes forward. He's very relieved that the death penalty is not on the table."

Bales allegedly left his base in the Panjwayi district of southern Kandahar province on the night of 11 March, 2012, to commit the killings. Nine of those killed were children. Bales allegedly set several of the bodies on fire.

At a pre-trial hearing in November, Bales's family insisted he was innocent until proven guilty, calling him "courageous and honorable," while his lawyer raised questions about the role of alcohol, drugs and stress in the tragedy.

But prosecutors lashed the "heinous and despicable" alleged massacre during an eight-day hearing.

Prosecutors at the so-called Article 32 pre-trial hearing alleged that Bales left the base twice to carry out the killings, returning in between and even telling a colleague what he had done.

The hearing included three evening sessions – daytime in Afghanistan – to hear testimony by video conference from Afghan victims and relatives of those who died.

In a statement read out by the soldier's sister Stephanie Tandberg after last year's hearing, the family said it had yet to learn the how, why and what of the incident.

"Much of the testimony was painful, even heartbreaking, but we are not convinced the government has shown us the truth, the whole truth and nothing but the truth about what happened that night," the statement said.  

"As a family, we all grieve deeply for the Afghani families who lost their loved ones on 11 March, but we must all not rush to judgment," it added. "In America, due process means innocence is always presumed unless and until a trial proves otherwise. There has been no trial yet, and our family member is presumed by law, and by us, to be innocent."