By Beth Stebner and Thomas Durante

Anar Gul gestures to the body of her grandchild, who was allegedly killed by a U.S. service member in Panjwai, Kandahar province south of Kabul, Afghanistan, Sunday, March. 11, 2012. Afghan President Hamid Karzai says a U.S. service member has killed more than a dozen people in a shooting including nine children and three women. (Photo: AP)

NATO troops in Afghanistan are on high alert after the Taliban vowed to avenge the deaths of 16 innocent civilians - including nine children and three women - who were shot and killed by a rogue U.S. soldier who opened fire after suffering a 'mental breakdown' early Sunday morning.

The Army staff sergeant, stationed at a U.S. base in Kandahar, entered three Afghan family’s homes at 3am and began the vicious killing spree. Relatives of the dead said he then 'poured chemicals over their dead bodies and burned them.'

The shooter is an Army staff sergeant from Fort Lewis-McChord in Washington state, and was believed to have acted alone.

Military officials are investigating the incident and working to discover what made the soldier - believed to be a father of three - snap to such extremes that he would embark on a killing mission.

With tensions rising in the region, U.S. and British officials said they were now braced for a backlash as the Taliban claimed the killings were the work of 'more than one soldier'.

Militants condemned the 'blood-soaked and inhumane crime' by 'sick-minded American savages' on its website and vowed to take revenge 'for every single martyr with the help of Allah'.

Disbelief: Two grief-stricken Afghan men look into the van where the body of a badly burned child lays, wrapped in a blue blanket. (Photo: EPA)

Horrific: The bodies of an elderly Afghan man and a child killed in the Alkozai village of Panjwayi district are shown wrapped in blankets. (Photo: AFP / Getty Images)

Initial reports indicated the gunman returned to his base after the shooting, calmly turned himself in and was taken into custody at a NATO base in Afghanistan.

In a statement, Afghan President Hamid Karzai left open the possibility of more than one shooter. He initially spoke of a single U.S. gunman, then referred to 'American forces' entering houses.

The statement quoted a 15-year-old survivor named Rafiullah, shot in the leg, as telling Karzai in a phone call that 'soldiers' broke into his house, woke up his family and began shooting them.

Mr Karzai condemned the attacks as 'an assassination' and furiously demanded an explanation from the U.S. He has also demanded the gunman be handed over so he can stand trial.

Little is known about the soldier who committed the atrocities, including his name, but a U.S. official said he is married with three children, and served three separate tours in Iraq.

He was assigned to support a special operations unit of either Green Berets or Navy SEALs engaged in a village stability operation.

Such operations are among NATO's best hopes for transitioning out of Afghanistan, pairing special operations troops with villagers chosen by village elders to become essentially a sanctioned, armed neighbourhood watch. He has reportedly been stationed in Afghanistan since December.

Fort Lewis-McChord is about 45 miles south of Seattle and home to about 100,000 military and civilian personnel.

A former soldier out of Fort Lewis shot and injured a Salt Lake City police officer in 2010, and on January 1, a 24-year-old Iraq War veteran shot and killed a Mount Rainier National Park ranger.

Tragedy: Locals people gather outside the houses where 17 civilians were murdered by a U.S. soldier in a horrific house-to-house killing spree. (Photo: KeystoneUSA-ZUMA / Rex Features)

Four Lewis-McChord soldiers were convicted in the deliberate thrill killings of three Afghan civilians in 2010. The military newspaper Stars and Stripes called it 'the most troubled base in the military' that year.

The attack is sure to further tarnish relations between Afghanistan and the U.S., as it comes weeks after NATO soldiers burned copies of the Koran - the Muslim holy book - sparking a violent protest that has left some 30 people dead.

And the former Commander of British Forces in Afghanistan, Colonel Richard Kemp, said British troops could now be targeted in revenge and that the massacre would also erode the vital trust allied forces have built up with Afghan civilians over the course of the war.

He told ITV's Daybreak: 'I think every soldier in Afghanistan, British, American and other allies, will be sickened by a person wearing their own uniform literally going door to door and killing people as they sleep in their houses.

'These are the very people that this soldier and his comrades are supposed to be in Afghanistan to protect not kill.

'You would have to make a very persuasive case that these actions were due to mental stress, that's not to say that the stress isn't there for every soldier in Afghanistan.'

Neighbours said they had awoken to crackling gunfire from American soldiers, who they described as laughing and drunk.

'They were all drunk and shooting all over the place,' said Agha Lala, who visited one of the homes where killings took place. 'Their [the victims'] bodies were riddled with bullets.'

A senior U.S. defence official in Washington rejected witness accounts that several apparently drunk soldiers were involved.

'Based on the preliminary information we have this account is flatly wrong,' the official said. 'We believe one U.S. service member acted alone, not a group of U.S. soldiers.'

Tears of grief: An Afghan youth mourns for his relatives, who were allegedly killed by the U.S. service member. (Photo: AP)

An AP photographer reported that he saw 15 bodies of Afghans - some of them burned and some covered with blankets - in the villages of Alkozai and Balandi in Kandahar province's Panjwai district.

One man told the AFP news agency of his great loss. ‘Eleven members of my family are dead. They are all dead,’ Haji Samad said.

'They [Americans] poured chemicals over their dead bodies and burned them,' a weeping Mr Samad told Reuters at the scene.

According to Al Jazeera, the soldier went into three separate houses at 3am local time when it was pitch black and shot the civilians, who were sleeping in their beds.

Perched: An elderly Afghan man sits next to the covered body of a person who was killed early today by a U.S. service member. (Photo: AP)

In shock: Relatives sat in shock in a van also carrying the bodies of their kin wrapped in blankets. (Photo: EPA)

A resident of Alkozai, where the shootings took place, said 16 people were killed as the U.S. service member went into three different houses and started shooting.

The villager, Abdul Baqi, said he had not seen the bodies himself, but had talked to the family members of the dead.

‘When it was happening in the middle of the night we were inside our houses. I heard gunshots and then silence and then gunshots again,’ Mr Baqi said.

Reports say that 15 members from two Afghan families were slaughtered, as well as an unidentified sixteenth person.

Mr Karzai also said that five people were wounded. Their conditions are unknown.

Defence Secretary Leon Panetta called the Afghan president to express ‘profound regret’ and assure him that ‘this terrible incident does not reflect our shared values or the progress we have made together,’ his office said in a statement.

He concluded: ‘We will bring those responsible to justice.’

Maj Jason Waggoner, another spokesman for ISAF said: ‘The civilian casualties were not the result of any operations. The soldier was acting on his own. After the incident, he returned to the compound and turned himself in.’

NATO-led International Security Assistance Force deputy commander Lt Gen Andrian Bradshaw would not speculate the reasoning behind the seemingly random attack.

Mr Karzai said in a statement that he was sending high-level authorities to investigate the shooting and deliver a full report. NATO officials, too, are conducting an inquiry.

‘This is an assassination, an intentional killing of innocent civilians and cannot be forgiven,’ Mr Karzai said in a statement, adding that he has repeatedly called for the U.S. to stop killing Afghan citizens.

A man sits in a truck bed keeping watch over the body of a young boy. (Photo: Reuters)

President Obama issued a statement this afternoon saying he is ‘deeply saddened’ by the ‘tragic and shocking’ killing of Afghan civilians by a U.S. soldier.

He said: ‘This incident is tragic and shocking and does not represent the exceptional character of our military and the respect that the United States has for the people of Afghanistan.'

The White House said that Mr Obama phoned Mr Karzai to personally express his regret.

The president also vowed to ‘get the facts as quickly as possible and to hold accountable anyone responsible.’

On Sunday, White House spokesman Josh Earnest said in a statement: ‘We are deeply concerned by the initial reports of this incident, and are monitoring the situation closely.’

There are precious few details on the alleged shooter. Officials have only said that he was an Army staff sergeant who was acting alone.

On CBS’ Face the Nation, Newt Gingrich commented on the escalating tensions in Afghanistan and elsewhere, saying: ‘I think that we have to reassess the entire region,’ noting Washington’s tumultuous relationship with neighbouring Pakistan as well.

Twelve of the dead were from Balandi, said Samad Khan, a farmer who lost all 11 members of his family, including women and children.

Mr Khan was away from the village when the incident occurred and returned to find his family members shot dead and burned.

One of his neighbours was also killed, he said. It was unclear how or why the bodies were set ablaze.

To prove that the bodies had been set on fire, Afghan villagers brought out badly burned blankets, the New York Times reported. More than 300 people came out to protest the senseless violence.

An AP photo showed the bloodstained corner of a house next to a large black area that was charred by fire. The charred area appeared to be remnants of blankets and possibly bodies that had been set on fire.

Villagers packed inside the minibus looked on with concern as a woman spoke to reporters. She pulled back a blanket to reveal the body of a smaller child wearing what appeared to be red pajamas.

A third dead child lay in a pile of green blankets in the bed of a truck.

'This is an anti-human and anti-Islamic act,' said Mr Khan. 'Nobody is allowed in any religion in the world to kill children and women.'

Mr Khan demanded that Karzai punish the American shooter.

'Otherwise we will make a decision,' said Mr Khan. 'He should be handed over to us,’ he told the Associated Press.

‘I cannot explain the motivation behind such callous acts, but they were in no way part of authorised ISAF military activity,’ he said in a statement.

There were reports of protests in Panjwai following the shooting and the U.S. embassy warned travellers in Kandahar province to ‘exercise caution.’

The Afghan Taliban would take revenge for the deaths, the group said in an e-mailed statement to media.

‘The so-called American peacekeepers have once again quenched their thirst with the blood of innocent Afghan civilians in Kandahar province,’ the Taliban’s statement read.

The shooting comes after weeks of tense relations between U.S. forces and their Afghan hosts following the burning of Korans and other religious materials at an American base.

Though U.S. officials apologised and said the burning was purely accidental, the incident sparked violent protests and attacks that killed some 30 people and a host of anti-American protests.

Six U.S. troops have been killed in attacks by their Afghan colleagues since news of the Koran burnings came to light.

Furious: Afghan civilians gathered outside the military base today; tensions between the U.S. and Afghanistan have been tense at best for the past decade. (Photo: AP)

In the capital, meanwhile, Mr Karzai said the government still expects to sign a strategic partnership agreement with the United States by the time a NATO summit convenes in Chicago in May.

The agreement would formalize the U.S.-Afghan relationship and the role of U.S. forces in Afghanistan after NATO's scheduled transfer of security responsibility to the Afghan government at the end of 2014.

But Mr Karzai stressed the importance of foreign forces leaving Afghanistan to preserve the country's national sovereignty.

Any international forces that remain after 2014 would have to operate under strict guidelines governing their responsibilities and when they could leave their bases, he said.

‘We have a strong army and police, so it is to our benefit to have good relations with the international community, not have international troops in our country,’ Mr Karzai said at a public event in Kabul.

The president has demanded that international forces stop night raids on the homes of suspected militants as a condition to signing the strategic partnership agreement.

The raids have caused widespread anger among Afghans.

All foreign combat troops are slated to withdraw by end of 2014 from a costly war that has become increasingly unpopular.

Malalai Joya

Also Sunday, a prominent Afghan women's rights activist said gunmen attacked her office in a western province in an apparent assassination attempt.

Malalai Joya, a former Afghan lawmaker and vocal critic of both the Taliban and of criminality in the Afghan government, said the attack on her office in Farah province was the sixth attempt on her life to date.

Armed men tried to storm the compound before dawn on Saturday, she said.

The attackers did not get into the building but two of her guards were seriously injured and are currently in the hospital.

Ms Joya said she was in Kabul at the time but had planned a trip to Farah soon and news of that may have leaked out.

She said she believes the attackers thought she was in the building.

The shooting spree took place in the Panjwai district of Kandahar province this morning.

Captain Justin Brockhoff, Nato spokesman confirmed a U.S. service member had been detained following the attack.

The wounded are receiving treatment at a medical base and U.S. forces are investigating the shooting with Afghan authorities.