A US service member has killed 16 people - including nine children and three women - in an Afghanistan shooting spree condemned by President Hamid Karzai as "an assassination".
Karzai demanded an explanation from the United States, adding new tensions to a relationship already severely strained over Americans burning Muslim holy books on a base in Afghanistan.
The burnings sparked violent protests and attacks that left some 30 people dead. Six US service members have been killed in attacks by their Afghan colleagues since the Quran burnings came to light, but the violence had 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 US stop killing Afghan civilians.
Five people were wounded in the pre-dawn attack in Kandahar province, including a 15-year-old boy named Rafiullah who was shot in the leg and spoke to the president over the telephone.
He described how the American soldier entered his house in the middle of the night, woke up his family and began shooting them, according to Karzai's statement.
Nato officials apologised 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 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.
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 pyjamas. A third dead child lay amid a pile of green blankets in the bed of a truck.
Nato spokesman Justin Brockhoff said a US 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 attack took place in two villages in the Panjwai district of southern Kandahar province. The villages - Balandi and Alkozai - are about 500 yards away from a US base.
The shooting started at about around 3 am local time, said Asadullah Khalid, the government representative for southern Afghanistan and a member of the delegation that went to investigate the incident.
A resident of the village of Alkozai, Abdul Baqi, said that, based on accounts of his neighbours, the American gunman went into three different houses and opened fire.
"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.
Karzai said he was sending a high-level delegation to investigate and deliver a full report.