SEATTLE - The attorney for the U.S. soldier accused of killing 17 Afghan civilians said Friday the U.S. government is "hiding evidence" from the soldier's defence team.

John Henry Browne told The Associated Press that members of the defence team in Afghanistan were told they would have access to witnesses at a hospital, but they later discovered the people had been released.

Browne also said the U.S. government has not turned over files to the lawyers defending Army Staff Sgt. Robert Bales.

The defence team said in a statement the prosecution is withholding information "while potential witnesses scatter."

"It's outrageous," Browne said. "What they are basically doing is hiding evidence. The only reason to hide evidence is if you don't have evidence."

An Army spokesman did not immediately return a call seeking comment on Browne's allegations.

Bales has been charged with 17 counts of premeditated murder. He remained held at a U.S. military prison at Fort Leavenworth, Kansas.

Dan Conway, a military attorney who represented a soldier convicted in the deliberate killings of three Afghan civilians in 2010, said the government doesn't appear to be doing anything wrong at this point in the Bales case. He said prosecutors have little obligation to turn over evidence right now and doesn't need to help co-ordinate interviews this early in the process.

"This is just going to be an uphill battle," Conway said.

The defence will have a right to interview witnesses who could be called at trial, so the Army could then take the defence team into the villages with security or co-ordinate to have them come onto the military base. But Conway said the challenges of interviewing witnesses now means the defence team may not be able to track down people to bolster their case -- such as witnesses unable to identify Bales or those who believe there were two shooters.

"If they want to talk to those witnesses, they're going to have to get an investigator and probably go to the village and talk to civilians themselves," Conway said.

U.S. military officials have said Bales was drinking on a southern Afghanistan base on March 11 before going to two villages at night, shooting his victims and setting many on fire. Nine were children.

The defence statement said the team in Afghanistan tried to interview injured civilians being treated at a hospital in Kandahar but were denied access and told to co-ordinate with prosecutors.

The prosecution team interviewed the civilians, but the defence team said they were unable to do so after the people were released, and no contact information was provided for them.

Browne's team also said they have been denied access to the civilians' medical records, as well as video allegedly taken from a surveillance blimp showing Bales on the night of the killings.