One of his lawyers said the Army staff sergeant agreed to plead guilty to premeditated murder, Reuters reported.
He would then appear before a jury that would decide if his life sentence would include parole.
“There will be a jury for the sentencing phase beginning in August,” Emma Scanlan said in an email to reporters.
Bales is the only suspect in two late-night killing sprees through Kandahar villages.
Prosecutors said he used a pistol, rifle and grenade launcher to attack people – mostly women and children – as they slept in their homes, according to Reuters.
Some of the bodies were stacked and burned. The attack spurned violent reaction in Afghanistan, the site of a long-running war between the US and its allies against the Taliban.
Bales is a decorated veteran of four combat tours in Iraq and Afghanistan. His defense lawyers say he suffered from post-traumatic stress and a head injury before deployment overseas.
Bales was twice injured in combat. He’s to be sentenced June 5 at Joint Base Lewis-McChord near Seattle, The Associated Press reported.
A judge and the base’s commanding general must approve the plea deal, the AP said.
“The judge will be asking questions of Sgt. Bales about what he did, what he remembers and his state of mind,” defense lawyer John Henry Browne told the AP.
Bales is a father of two originally from Ohio. He was supposedly drinking and had been taking drugs – Valium and steroids – before the attacks, the AP said.