BALANDI, Afghanistan (AP) — An American soldier opened fire on villagers near his base in southern Afghanistan Sunday and killed 16 civilians, according to President Hamid Karzai, who called it an "assassination" and furiously demanded an explanation from Washington. Nine children and three women were among the dead.

The killing spree deepened a crisis between U.S. forces and their Afghan hosts over Americans burning Muslim holy books on a base in Afghanistan last month. The Quran burnings sparked weeks of violent protests and attacks that left some 30 dead. Six U.S. service members have been killed by their Afghan colleagues since the burnings came to light, and the violence had just started to calm down.

"This is an assassination, an intentional killing of innocent civilians and cannot be forgiven," Karzai said in a statement. He said he has repeatedly demanded the U.S. stop killing Afghan civilians.

President Barack Obama phoned Afghan President Hamid Karzai to express his shock and sadness at the killing and wounding of Afghan civilians. He offered condolences to the grieving families of those killed 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.

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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.

Sunday's shooting could push that agreement further away.

"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.

"This may have been the act of a lone, deranged soldier. But the people of Afghanistan will see it for what it was, a wanton massacre of innocent civilians," Cortright said.

The attack began around 3 a.m. in two villages in Panjwai district, a rural suburb of Kandahar and a traditional Taliban stronghold where coalition forces have fought for control for years. The villages — Balandi and Alkozai — are about 500 yards (meters) from a U.S. base.

The gunman went into three houses and opened fire, said a resident of Alkozai, Abdul Baqi, citing accounts from his neighbors.

"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," Baqi said.

One villager said eleven of those killed were members of his family, many of them women and children.

An AP photographer saw 15 bodies in the two villages caught up in the shooting. Some of the bodies had been burned, while others were 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.

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.

A U.S. official in Washington said the American, an Army staff sergeant, was believed to have acted alone and that initial reports indicated he returned to the base after the shooting and turned himself in.

However Karzai's statement left some confusion on the point. He spoke of a single U.S. gunman, but in another part of the statement referred to "American forces" entering the houses.

It also said the president spoke by phone to one of the five people wounded in the attack, a 15-year-old named Rafiullah, who was shot in the leg. The statement said the teenager told Karzai that American soldiers entered his house in the middle of the night, woke up his family and began shooting them.

NATO officials apologized for the shootings but did not confirm that anyone was killed, referring instead to reports of deaths.

"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.

He pledged a "rapid and thorough investigation" and vowed to ensure that "anyone who is found to have committed wrongdoing is held fully accountable."

NATO spokesman Justin Brockhoff said a U.S. service member had been detained at a NATO base as the alleged shooter. The wounded people were evacuated to NATO medical facilities, he added.

International forces have fought for control of Panjwai for years as they've tried 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 Omar, were born, raised, fought or preached in the area. Omar once ran an Islamic school in an area of Panjwai that has since been carved into a new district.

In addition to its symbolic significance, the district is an important base for the Taliban to target the city of Kandahar to the east. Panjwai was seen as key to securing Kandahar when U.S. forces flooded the province as part of Obama's strategy to surge in the south starting in 2009.

Twelve of the dead were from Balandi, 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 incident 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," said Khan. "Nobody is allowed in any religion in the world to kill children and women."

Khan and other villagers demanded that Karzai punish the American shooter.

"Otherwise we will make a decision," said Khan. "He should be handed over to us."

The four people killed in the village of Alkozai were all from one family, said a female relative who was shouting in anger. She did not give her name because of the conservative nature of local society.

"No Taliban were here. No gunbattle was going on," said the woman. "We don't know why this foreign soldier came and killed our innocent family members. Either he was drunk or he was enjoying killing civilians."

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

"The so-called American peace keepers 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.

Karzai said he was sending a high-level delegation to investigate.

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.

And in January, before the Quran burning incident, a video that purportedly showed U.S. Marines urinating on corpses of men they had killed sparked widespread outrage.

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.

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Vogt reported from Kabul, Afghanistan. Associated Press writers Sebastian Abbot and Rahim Faiez in Kabul and Lolita C. Baldor in Washington contributed to this report. AP photographer Allauddin Khan contributed from Balandi.