A US soldier accused of killing 16 people in southern Afghanistan during a shooting spree acted alone, NATO said. The soldier\'s motive in the Sunday shootings, which Afghan President Hamid Karzai said were \"acts of terror and unforgivable,\" remained unclear. The Los Angeles Times quoted a US official as saying the assailant, who turned himself in after the shooting, apparently suffered a mental breakdown. Afghan officials said the soldier, an Army staff sergeant, left his base early Sunday and went from house to house, breaking in doors and killing people. The incident occurred in Panjwai district in southern Kandahar province, a Taliban stronghold. The dead included three women and nine children. Five more people were wounded. Haji Agha Lali, a member of the Kandahar provincial council, told CNN the soldier had attacked four houses in two nearby villages. Capt. Justin Brockhoff, spokesman for NATO\'s International Security Assistance Force, said the wounded Afghans were being treated at ISAF facilities. He said both NATO and Afghan officials were trying to determine the soldier\'s motive, while another spokesman said the soldier was acting on his own, CNN reported. A US military official told CNN the suspect is from Joint Base Lewis-McChord in Washington state and that he had been assigned to a Special Forces unit. Capt. John Kirby, an ISAF spokesman, said while it was not clear what led to the shootings, ISAF Commander Gen. John Allen has made it clear the investigation would be thorough. The incident comes at a time when the situation in Afghanistan remains tense over the inadvertent Koran-burning by NATO soldiers at a base, which triggered protests that have claimed the lives of more than 30 Afghans and six US soldiers. U.S. President Barack Obama called Karzai on Sunday and said the latest killings were \"tragic and shocking,\" and offered his condolences to the Afghan people, the White House said. However, the latest attack is likely to further aggravate the situation, CNN reported. \"The Afghan people can withstand a lot of pain,\" Prince Ali Seraj, the head of the National Coalition for Dialogue with the Tribes of Afghanistan, told CNN. \"They can withstand collateral damage. They can withstand night raids. But murder is something that they totally abhor, and when that happens, they really want justice.\" Obama said the US military will \"get the facts as quickly as possible and to hold accountable anyone responsible.\" \"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. Defense Secretary Leon Panetta and acting U.S. Ambassador James Cunningham also expressed their condolences. The Washington Post said although the suspect was immediately detained, the shootings appeared certain to stir up anti-American sentiments in Afghanistan and raise more questions among Americans about the US mission in that country. The US troops are set to end their combat mission in Afghanistan by the end of 2014. \"My fear is that those Afghans in the region that were indifferent to either side of this conflict will now, at least as a temporary emotional reaction, become active insurgents,\" a US Army officer in Kandahar told the Post. A Taliban statement called the incident a massacre. \"The so-called American peace keepers have once again quenched their thirst with the blood of innocent Afghan civilians,\" the statement said. A commentary in Britain\'s Independent newspaper warned the Koran burnings and the latest shootings are an \"immense propaganda boost to the insurgents and add to the alienation of the population, many of whom are susceptible to conspiracy theories of foreign forces secretly engineering these treacherous acts.\" A New York Times analysis said the two incidents and the outrage stemming from them could adversely affect the Obama administration\'s plans to speed up the training of Afghan forces so they can take the lead in combat missions, while seeking to bring the Taliban into negotiations to end the decade-long war. The report quoted some American and civilian officials as saying the latest events would embolden the Taliban hard-liners who oppose negotiations, since they see the foreign forces would be leaving the country in any case. \"The fear is that all these incidents, taken together, play into the Taliban\'s account of how we treat the Afghan religion and people,\" one official said. Benjamin Rhodes, deputy national security adviser for strategic communications, told the Times in an interview that while the Koran-burning and the Sunday shootings are \"heart-wrenching, very difficult moments,\" the United States learned during the Koran burning that \"if you respond appropriately, you can actually build trust with the Afghans.\"