The US soldier accused of killing 16 Afghan villagers was upset by a serious injury sustained by an army comrade the day before the massacre, but held no animosity toward Muslims, his lawyer has said.

The 38-year-old army sergeant, being held in Kuwait, was also unhappy about being sent to Afghanistan, having earlier been told he would not be redeployed after three tours in Iraq, according to his Seattle-based civilian attorney.

The soldier, a married father-of-two is not being named, and his family has moved to a military base south of Seattle because of security fears and concerns about possible retaliation for the attack.

A decorated veteran, the suspect is alleged to have left his base in southern Kandahar province before dawn Sunday and then proceeded to kill the Afghans, many of them children, in two neighboring villages.

"We have been informed that at this small base that he was at, somebody was gravely injured the day before the alleged incident, which affects all of the soldiers there," attorney John Henry Browne said in Seattle on Thursday.

He declined to give more details to reporters, but a report said a friend and fellow US soldier had his leg blown off while next to the rampage suspect.

The New York Times said the suspect, a career soldier originally from the Midwest, had been drinking at the time of the shootings, was stressed over his fourth deployment and had been experiencing tensions with his wife.

"When it all comes out, it will be a combination of stress, alcohol and domestic issues -- he just snapped," an unnamed official told the Times.

But the Seattle lawyer, who has taken on high-profile cases before, questioned the report, saying there were no marital problems although stress could be a factor.

"I don't have any information that alcohol is a factor," he said. "I imagine stress is a factor. Who's not going to be under stress in Afghanistan, in a small camp where there's 20 people, in the middle of nowhere?" he asked.

Browne added: "I questioned the family members and he had no animosity towards Muslims."

He said his client, who left for Afghanistan in December, had earlier been told he would not be deployed there after being in Iraq, where he suffered head and foot injuries.

"The family was counting on him not being redeployed, and then literally overnight that changed," Browne said.

"So I think that it would be fair to say that he and the family were not happy that he was going back."

The suspect has been flown to Kuwait, in a first step towards being charged and put on trial outside Afghanistan, defying demands by Afghan leaders for a public trial at home.

Browne said it was possible the suspect could be tried in a military court at Joint Base Lewis-McChord, south of Seattle, which has been the soldier's home base throughout his military career.

But he could equally be tried in any major US garrison, or possibly in a military court outside the country, but not in Afghanistan.

"That's up in the air at the moment," Browne said, but adding that he thought legal proceedings in Afghanistan were "completely off the table."

Browne is known for representing Colton Harris-Moore, the so-called "Barefoot Bandit," who was found guilty of a string of audacious crimes across the United States and sentenced in January to six-and-a-half years in prison.