A decorated US soldier who shot dead 16 unarmed Afghan civilians last year has apologised at a sentencing hearing, calling the killings "an act of cowardice".

Army Staff Sergeant Robert Bales has admitted to shooting the villagers, mostly women and children, in attacks on their family compounds in Kandahar province in March 2012.

"I am truly, truly sorry to all the people whose family members I have taken away. I have murdered their families," Bales told a jury in a military courtroom in Washington state.

"What I did is an act of cowardice."

Bales, a veteran of four combat tours in Iraq and Afghanistan, pleaded guilty to the killings in June in a deal that will spare him the death penalty.

A jury of six military personnel will ultimately decide if he will spend the rest of his life in prison or be eligible for parole after 20 years.

Closing arguments are due on Friday.

Defence lawyers have been seeking to show Bales suffered a breakdown under the pressure of his final deployment to Afghanistan.

They have argued that he suffered from post-traumatic stress disorder and a brain injury even before his last deployment to Afghanistan.

Speaking slowly and deliberately, Bales told the jury he had struggled with anger issues that worsened after his third military deployment.

He said trivial issues such as dirty dishes in the sink would make him angry, and that he was "mad at myself for being mad".

He briefly sought treatment between his third and fourth deployments, but felt it was not working and stopped.

"I was a coward for stopping. If you have a problem and you seek help, it's not weakness, it's strength," he said.

Under military court rules, he was not subject to cross-examination.