President Obama and U.S. officials offered their condolences for the alleged shooting of civilians by a U.S. service member in Afghanistan amid fears the incident could trigger another wave of anti-American violence in the region and fuel a push to abandon the war-worn country.
Tensions between the U.S. and Afghanistan had appeared to be easing after Americans burning Korans last month sparked deadly protests across the country.
"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 Koran 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.
Residents said 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 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 stalked house after 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.
Pentagon spokesman Capt. John Kirby told Fox News the American soldier is in his mid-30's and had been deployed to Iraq twice before his first mission in Afghanistan. The soldier is reportedly married with two children.
President Obama issued condolences after the incident, saying he is "deeply saddened" by the attacks.
U.S. officials briefed Obama earlier in the day after an unnamed soldier was detained by his unit following an alleged targeting of as many as 16 Afghan civilians in two neighboring villages.
"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," Obama said in a statement. "I offer my condolences to the families and loved ones of those who lost their lives, and to the people of Afghanistan, who have endured too much violence and suffering.
The president said he also supports a military investigation to "get the facts as quickly as possible and hold accountable anyone responsible."
"We are deeply concerned by the initial reports of this incident, and are monitoring the situation closely," White House Deputy Press Secretary Josh Earnest said.
Pentagon spokesman Capt. John Kirby told Fox News the suspect is currently being interviewed and promises a thorough but "very rapid" investigation.
"We're going to let the facts and the evidence take us wherever they need to take us to figure out exactly what happened," Kirby said.
Motive for the attacks has yet to be determined.
Defense Secretary Leon Panetta said an investigation is already underway and a suspect is in custody.
"I gave (Afghan) President (Hamid) Karzai my assurances that we will bring those responsible to justice," Panetta said in a statement. "We will spare no effort in getting the facts as quickly as possible, and we will hold any perpetrator who is responsible for this violence fully accountable under the law."
NATO officials apologized for the shootings but did not confirm that anyone was killed, referring instead to reports of deaths.
"I wish to convey my profound regrets and dismay at the actions apparently taken by one coalition member in Kandahar province, said a statement from Lt. Gen. Adrian Bradshaw, the deputy commander of NATO forces in Afghanistan.
"One of our soldiers is reported to have killed and injured a number of civilians in villages adjacent to his base. I cannot explain the motivation behind such callous acts, but they were in no way part of authorized ISAF military activity," he said, using the abbreviation for NATO's International Security Assistance Force.
Reports are conflicting as to the number dead as Afghan villagers showed to press photographers bodies covered in blankets as well as burned. Nine children and three women are said to be among the dead. It was unclear how or why the bodies were burned.
The incident comes after some very tense weeks in which riots raged over the accidental improper disposal of Korans that had been defaced by Afghan detainees.
The top commander in Afghanistan, Gen. John R. Allen, issued a statement saying that he is "absolutely dedicated to making sure that anyone who is found to have committed wrongdoing is held fully accountable."
The head of the ISAF added that he was "shocked and saddened to hear" about the incident and offered his "profound regret and deepest condolences to the victims and their families."
"I pledge to all the noble people of Afghanistan my commitment to a rapid and thorough investigation," Allen said, adding that the U.S. will maintain custody of the service member, provide medical care to injured Afghans and "cooperate fully with local Afghan authorities as we ascertain all the facts."
"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. Nor does it impugn or diminish the spirit of cooperation and partnership we have worked so hard to foster with the Afghan National Security Forces," he added.
The U.S. Embassy in Kabul tweeted videotaped condolences in the primary languages of Pashto and Dari. It also warned Americans in the country to beware of the "risk of anti-American feelings and protests in coming days, especially in eastern and southern provinces."
"U.S. citizens in Afghanistan should remain vigilant and avoid areas where Westerners congregate. Avoid large public gatherings or demonstrations. Past demonstrations in Afghanistan have escalated into violent attacks on Western targets of opportunity.
"Do not discuss travel plans or other personal matters with strangers, or in public. Be alert and aware of your surroundings, and always travel with mobile phones or appropriate communication equipment," the warning reads.
The alleged shootings are likely to spark more violence after a calm returned following the Koran burnings of last month. That accidental incident led to more than a week of rioting, which resulted in 30 people dead, including six U.S. service members who were killed by their Afghan colleagues.
The killing of U.S. service members by Afghan forces had already been a major concern in Afghanistan, where since May 2007, 46 "green on blue" attacks -- or Afghan National Security Forces attacks on allied service members -- has reportedly resulted in 76 NATO soldiers killed and an undisclosed number wounded.
Karzai called the attacks an "assassination" and angrily demanded an explanation. Karzai said he spoke with a 15-year-old boy named Rafiullah who was shot in the leg during the incident. The boy described how an American soldier entered his house in the middle of the night, woke up his family and began shooting them.
"This is an assassination, an intentional killing of innocent civilians and cannot be forgiven," Karzai said in a statement that is sure to incite more violence.
As violence erupts, U.S. officials have debated a faster exit strategy from Afghanistan, which is supposed to be left to fend for itself by 2014. In anticipation of that departure, the U.S. and Afghan governments signed a memorandum of understanding recently 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.
Sen. John McCain, the ranking Republican on the Senate Armed Services Committee, said the alleged incident cannot be explained "except to extend your deepest sympathy to those victims and see that justice is done."
But, he added, the U.S. is in Afghanistan because that's where the attacks of Sept. 11, 2001, originated.
"If Afghanistan dissolved into a situation where the Taliban were able to take over or a chaotic situation, it could easily return to an Al Qaeda base for attacks on the United States of America. That is still our goal, as it was the day we went in," McCain said.
Panjawi, the town where the alleged shootings occurred, is inside a rural Taliban stronghold, and is just south of the birthplace of the militant group's senior leaders, including chief Mullah Omar. Panjawi is a launching point for attacks on neighboring Kandahar City and is a key to securing the province, one of the most difficult battlegrounds in the country.
The Taliban issued a statement on an insurgent website expressing concern for the villagers.
"The so-called American peacekeepers have once again quenched their thirst with the blood of innocent Afghan civilians in Kandahar province," said the statement.
The Associated Press contributed to this report.