LOS ANGELES (AFP) - A US soldier accused of killing 16 Afghan villagers last year will plead guilty in exchange for escaping the death penalty, his lawyer said.

Sergeant Robert Bales is due to face a court-martial in September on charges including 16 counts of murder, six of attempted murder and seven of assault over the killings in southern Afghanistan in March last year.

Seventeen of the 22 victims were women or children and almost all were shot in the head.

His lawyer John Browne was cited by The New York Times and other media on Wednesday as saying military prosecutors had agreed to the guilty plea deal, which could be made before a military judge next week.

A hearing is scheduled for next Wednesday at Joint Base Lewis-McChord in the northwestern US state of Washington, where Bales is being held.

"Bales is expected to enter a plea during this hearing," JBLM spokesman Gary Dangerfield said in a statement.

The 39-year-old allegedly left his base in the Panjwayi district of southern Kandahar province on the night of March 11, 2012, to commit the killings, which included nine children. Bales allegedly set several of their bodies on fire.

At a pre-trial hearing in November, Bales's family insisted he was innocent until proven guilty, calling him "courageous and honourable," 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, details of which were given at the military base south of Seattle.

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," it said.

"As a family, we all grieve deeply for the Afghani families who lost their loved ones on March 11, but we must all not rush to judgment.

"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."