The US soldier accused of shooting dead 16 Afghans had received body and brain injuries while serving in Iraq and was unhappy about going for another tour of duty, a lawyer has said.

John Henry Browne said the 38-year-old soldier, who has not been named, had already completed three tours in Iraq.

He also said the accused had witnessed his friend's leg blown off the day before the killings.

Sunday's shootings have placed new strains on the US in Afghanistan.

Meanwhile, a Nato helicopter carrying Turkish troops has crashed into a house on the outskirts of the capital Kabul killing at least 12 soldiers and two children on the ground.

The death toll is the heaviest single loss of life so far for Turkish troops in Afghanistan, of whom there are currently more than 1,800.

Police told the BBC a technical fault was to blame.

The Taliban called off peace talks in the wake of the deadly rampage - in which men, women and children were shot and killed at close range - although they made no mention of the massacre in the statement.

However, the US later stressed it remained committed to Afghan reconciliation despite the move by the Taliban.

Afghan President Hamid Karzai has also told the US that it must pull back its troops from village areas and allow Afghan security forces to take the lead in an effort to reduce civilian deaths.

'Treat him fairly'

Speaking in Seattle, where the accused soldier is based at Joint Base Lewis-McChord, Mr Browne said his new client was a "mild-mannered" man who bore no antipathy towards Muslims.

Previous tension points February 2012: Violent protests erupt after US troops inadvertently burn copies of the Koran at Bagram air base in Kabul. At least 30 are killed

Violent protests erupt after US troops inadvertently burn copies of the Koran at Bagram air base in Kabul. At least 30 are killed January 2012: US and UN officials describe a video clip of US marines urinating on dead Afghans as "disgusting" and "inhuman"

US and UN officials describe a video clip of US marines urinating on dead Afghans as "disgusting" and "inhuman" April 2011: US President Barack Obama describes March 2011 Koran burning by a radical US pastor as "intolerance and bigotry". The incident triggered protests which left at least 24 people dead in Afghanistan

US President Barack Obama describes March 2011 Koran burning by a radical US pastor as "intolerance and bigotry". The incident triggered protests which left at least 24 people dead in Afghanistan April 2008:Dutch and Danish governments evacuate their embassies in Kabul after protests against cartoon of the Prophet Muhammad which was reprinted by Danish newspapers In pictures: Kandahar shootings

He described him as "a decorated soldier" with an exemplary record before the shooting.

The lawyer also suggested the soldier, who began his first deployment to Afghanistan in December, was not fit to serve because of injuries he had suffered on previous tours of duty.

Mr Browne said that a car accident caused by a roadside bomb in Iraq had given the soldier a concussive head injury. And part of his foot had been removed in surgery because of a battle-related wound, he added.

"I think it's of interest that we have a soldier who has an exemplary record, a decorated soldier who was injured in Iraq, to his brain and to his body and then despite that was sent back," he said.

Mr Browne, who has represented a number of high-profile clients including serial killer Ted Bundy and a teenage thief known as the Barefoot Bandit, said his client was a happily married man with two children, aged three and four.

He denied reports that the accused had problems either with alcohol or his marriage, which Mr Browne described as "fantastic".

The US military has not yet charged the soldier, who has been in the military for 11 years, and Mr Browne said he would not release the accused's name until it was made public by officials.

However, despite the shock of the killings, Mr Browne called for calm and for the soldier to receive a fair hearing.

"It's a tragedy all the way round, there's no question about that.

"I think the message for the public in general is that he's one of our boys and they need to treat him fairly."

'Snapped'

Separately, an un-named US military officialtold the New York Timesthat the suspect had simply "snapped", and confirmed that he was on his fourth combat tour.

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

The soldier was flown out of Afghanistan on Wednesday and taken to a US military base in Kuwait.

According to the New York Times, he may be moved to a US facility, such as Fort Leavenworth, Kansas, as early as Friday.

The newspaper said Kuwaiti protests over the presence of the accused soldier within their borders had prompted the US to move him quickly.

The attack, in Kandahar province, was the latest in a string of damaging incidents involving US troops in Afghanistan.

Last month, the burning of Korans by troops sparked a string of deadly protests and violence in which at least 30 Afghans and six US soldiers were killed.

The latest attack has also caused uproar, with about 2,000 people demonstrating in the southern province of Zabul on Thursday, the second major protest outside Kandahar this week.