A US soldier charged with capital murder in the slayings of 16 civilians near his military post in Afghanistan was diagnosed as suffering from post-traumatic stress disorder (PTSD) and traumatic brain injury prior to the killings, his lawyer has claimed.

The disclosure that Robert Bales was diagnosed with PTSD followed a hearing on Thursday in which defence lawyers told a military judge they were preparing a possible "mental health defence".

Prosecutors are seeking the death penalty against Bales, a decorated veteran of four combat tours in Iraq and Afghanistan who is accused of gunning down the villagers, mostly women and children, in cold blood during two rampages through their family compounds in Kandahar province in March of last year.

The judge, Colonel Jeffery Nance, said such a defence would require a formal psychiatric evaluation and he would order a "sanity board" of independent doctors to review Bales' mental condition.

Defence lawyers also deferred entering a plea on behalf of their client and waived a formal reading of the charges.

Under the military justice system, a plea is commonly postponed at this stage to preserve legal options for the defence, whose ability to make additional motions is severely restricted once a plea is entered, experts say.

Civilian defence lawyer John Henry Browne told the judge, Colonel Jeffery Nance, that he would need at least a year and a half to prepare for Bales' defence.

IED Blast in Iraq

Prosecutors say Bales, a father of two from Lake Tapps, Washington, acted alone and with "chilling premeditation" when, armed with a pistol, a rifle and a grenade launcher, he left his base twice, returning in the middle of his rampage to tell a fellow soldier: "I just shot up some people."

The shootings, which occurred over a five-hour period in March, marked one of the deadliest incidents the military has blamed on a rogue US soldier since the Vietnam War, and strained US-Afghan relations.

Browne told reporters he doubted Bales would get a fair trial unless "we slow this thing down".

He said two Afghans that prosecutors had listed as potential witnesses in the case turned out to be insurgents who were killed by US-led forces, a claim that could not be immediately corroborated with US military officials.

Browne said he had government documentation showing that personnel at Lewis-McChord's Madigan Medical Center had found his client to be suffering from both post-traumatic stress disorder and a traumatic brain injury.

Defence lawyers previously have said Bales had suffered a possible concussion from a bomb blast during a prior tour of duty in Iraq.

Bales was bound over for court-martial in December and faces 16 murder charges, as well as other charges, including attempted murder, assault and drug and alcohol charges.

During a pre-trial hearing in November witnesses testified that he had been angered by a bomb blast near his outpost that

severed a fellow soldier's leg days before the shootings.

The government believes Bales was solely responsible for the deaths, and survivors have testified that they saw only one US soldier. However, several indirect accounts have suggested more than one soldier may have been involved.