Moving from house to house, a U.S. Army sergeant opened fire Sunday on Afghan villagers as they slept, killing 16 people _ mostly women and children _ in an attack that reignited fury at the U.S. presence following a wave of deadly protests over Americans burning Qurans.

The attack threatened the deepest breach yet in U.S.-Afghan relations, raising questions both in Washington and Kabul about why American troops are still fighting in Afghanistan after 10 years of conflict and the killing of Osama bin Laden.

The slayings, the worst atrocity committed by U.S. forces during the Afghan war, came amid deepening public outrage spurred by last month's Quran burnings and an earlier video purportedly showing American Marines urinating on dead Taliban militants.

The Quran burnings sparked weeks of violent protests and attacks that left some 30 Afghans dead, despite an apology from President Barack Obama. Six U.S. service members were also killed by their fellow Afghan soldiers, although the tensions had just started to calm down.

According to U.S. and Afghan officials, Sunday's attack began around 3 a.m. in two villages in Panjwai district, a rural region outside Kandahar that is the cradle of the Taliban and where coalition forces have fought for control for years. The villages are about 500 yards (meters) from a U.S. base in a region that was the focus of Obama's military surge strategy in the south starting in 2009.

Villagers described cowering in fear as gunshots rang out as a soldier roamed from house to house firing on those inside. They said he entered three homes in all and set fire to some of the bodies. Eleven of the dead were from a single family, and nine of the victims were children.

U.S. officials said the shooter, identified as an Army staff sergeant, acted alone, leaving his base in southern Afghanistan and opening fire on sleeping families in two villages. Initial reports indicated he returned to the base after the shooting and turned himself in. He was in custody at a NATO base in Afghanistan.

The suspect, from Joint Base Lewis-McChord, Wash., was assigned to support a special operations unit of either Green Berets or Navy SEALs engaged in a village stability operation, said a U.S. official, speaking on condition of anonymity because the investigation is still ongoing.

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 neighborhood watch.

Some residents said they believed there were multiple attackers, given the carnage.

``One man can't kill so many people. There must have been many people involved,'' Bacha Agha of Balandi village told The Associated Press. ``If the government says this is just one person's act we will not accept it. ... After killing those people they also burned the bodies.''

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, who was 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.

``This is an assassination, an intentional killing of innocent civilians and cannot be forgiven,'' Karzai said.

Obama phoned the Afghan leader to express his shock and sadness, and offered condolences to the grieving families and to the people of Afghanistan.

In a statement released by the White House, Obama called the attack ``tragic and shocking'' and not representative of ``the exceptional character of our military and the respect that the United States has for the people of Afghanistan.'' He vowed ``to get the facts as quickly as possible and to hold accountable anyone responsible.''

The violence over the Quran burnings had already spurred calls in the U.S. for a faster exit strategy from the 10-year-old Afghan war. Obama even said recently that ``now is the time for us to transition.'' But he also said he had no plan to change the current timetable that has Afghans taking control of security countrywide by the end of 2014.

In the wake of the Quran burnings, the top U.S. commander in Afghanistan, Gen. John Allen, visited troops at a base that was attacked last month and urged them not to give in to the impulse for revenge.

The tensions between the two countries had appeared to be easing as recently as Friday, when the two governments signed a memorandum of understanding about the transfer of Afghan detainees to Afghan control _ a key step toward an eventual strategic partnership to govern U.S. forces in the country.

Now, another wave of anti-American hatred could threaten the entire future of the mission, fueling not only anger among the Afghans whom the coalition is supposed to be defending but also encouraging doubts among U.S. political figures that the long and costly war is worth the sacrifice in lives and treasury.

``This is a fatal hammer blow on the U.S. military mission in Afghanistan. Whatever sliver of trust and credibility we might have had following the burnings of the Quran is now gone,'' said David Cortright, the director of policy studies at Notre Dame's Kroc Institute for International Peace Studies and an advocate for a quick withdrawal from Afghanistan.

Gen. Allen offered his regret and ``deepest condolences'' to the Afghan people for the shootings and vowed to make sure that ``anyone who is found to have committed wrongdoing is held fully accountable.''

``This deeply appalling incident in no way represents the values of ISAF and coalition troops or the abiding respect we feel for the Afghan people,'' Allen said in a statement, using the abbreviation for NATO's International Security Assistance Force.

In Panjwai district on Sunday, grieving residents tried to make sense of why they were targeted.

``No Taliban were here. No gunbattle was going on,'' cried out one woman, who said four people were killed in the village of Alokzai, all members of her family. ``We don't know why this foreign soldier came and killed our innocent family members. Either he was drunk or he enjoyed killing civilians.''

The other 12 dead were from Balandi village, said Samad Khan, a farmer who lost all 11 members of his family, including women and children. Khan was away from the village when the attack occurred and returned to find his family members shot and burned. One of his neighbors was also killed, he said.

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

One woman opened a blue blanket with pink flowers to reveal the body of her 2-year-old child, who was wearing a blood-soaked shirt.

``Was this child Taliban? There is no Taliban here'' said Gul Bushra. The Americans ``are always threatening us with dogs and helicopters during night raids.''

Dozens of villagers crowded the streets as minibuses and trucks carried away the dead to be washed for burial. One man used the edge of his brown shawl to wipe away tears.

Officials wearing white plastic gloves picked up bullet casings from the floor of a house and put them in a plastic bag.

An AP photographer saw 15 bodies in the two villages, some of them burned and other covered with blankets. A young boy partially wrapped in a blanket was in the back of a minibus, dried blood crusted on his face and pooled in his ear. His loose-fitting brown pants were partly burned, revealing a leg charred by fire.

It was unclear how or why the bodies were burned, though villagers showed journalists the blood-stained corner of a house where blankets and possibly bodies were set on fire.

International forces have fought for control of Panjwai for years, trying to subdue the Taliban in their rural strongholds. The Taliban movement started just to the north of Panjwai and many of the militant group's senior leaders, including chief Mullah Mohammed Omar, were born, raised, fought or preached in the area.

The district has also been a key Taliban base for targeting neighboring Kandahar city and U.S. forces flooded the province as part of Obama's strategy to surge in the south starting in 2009.

The Taliban called the shootings the latest sign that international forces are working against the Afghan people.

``The so-called American peacekeepers have once again quenched their thirst with the blood of innocent Afghan civilians in Kandahar province,'' the Taliban said in a statement posted on a website used by the insurgent group.

U.S. forces have been implicated before in other violence in the same area.

Four soldiers from a Stryker brigade out of Lewis-McChord, Washington, have been sent to prison in connection with the 2010 killing of three unarmed men during patrols in Kandahar province's Maiwand district, which is just northwest of Panjwai. They were accused of forming a ``kill team'' that murdered Afghan civilians for sport _ slaughtering victims with grenades and powerful machine guns during patrols, then dropping weapons near their bodies to make them appear to have been combatants.

Obama has apologized for the Quran burnings and said they were a mistake. The Qurans and other Islamic books were taken from a detention facility and dumped in a burn pit last month because they were believed to contain extremist messages or inscriptions. A military official said at the time that it appeared detainees were exchanging messages by making notations in the texts. (AP)