Balandi, Afghanistan: Fifteen Afghans were killed by an American soldier who walked off his base and opened fire on them in their homes in the early hours on Sunday, Afghan and western sources said.
Villagers showed 15 bodies, including women and children, and said they were all killed by the American.
The shooting could deepen strife between US forces and their Afghan hosts just as weeks of violence set off by the burning of copies of Quran at a US base had started to die down.
The burnings sparked violent protests and attacks that killed some 30 people. Six US service members have been killed in attacks by their Afghan colleagues since the Quran burnings came to light.
Nato officials apologised for Sunday's 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 Lieutenant General 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 authorised ISAF military activity," he said, using the abbreviation for Nato's International Security Assistance Force.
An AP photographer saw 15 bodies between 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.