President Hamid Karzai said a U.S. service member killed 16 people - nine of them children and three women - in a shooting spree Sunday.
Villagers showed an Associated Press photographer 15 bodies, including women and children, and alleged they were killed by the American.
"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. Karzai said he was sending a high-level delegation to investigate and deliver a full report.
Witnesses say the soldier walked into three different houses and started shooting.It's not known if the shooter knew the victims. 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.
President Barack Obama phoned 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."
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," said the top U.S. commander in Afghanistan, Gen. John Allen, using the abbreviation for NATO's International Security Assistance Force.
Allen pledged a "rapid and thorough investigation" and vowed to ensure that "anyone who is found to have committed wrongdoing is held fully accountable."
The shooting happened in two villages in the Panjwai district of southern Kandahar province - Balandi and Alkozai. The villages are about 500 yards away from a U.S. base.
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.
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.
The Associated Press contributed to this report.