BALANDI, Afghanistan (AP) — President Hamid Karzai said a U.S. service member killed 16 people — nine of them children and three women — in a shooting spree Sunday that he condemned 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 U.S. service members have been killed in attacks by their Afghan colleagues since the Quran burnings came to light, but the violence has just 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 U.S. 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 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.

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. His loose-fitting brown pants were partly burned, revealing a leg charred by fire.

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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 pajamas. A third dead child lay amid a pile of green blankets in the bed of a truck.

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 attack took place in two villages in the Panjwai district of southern Kandahar province. The villages — Balandi and Alkozai — are about 500 yards (meters) away from a U.S. base. The shooting started around 3 a.m., 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, told the AP that, based on accounts of his neighbors, 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.

International forces have fought for control of Panjwai for years as they've tried to subdue the Taliban in their rural strongholds. The Taliban movement started just to the north of Panjwai and the district was seen as key to securing Kandahar city to east when U.S. forces flooded the province as part of President Barack Obama's strategy to surge in the south starting in 2009.

Karzai said he was sending a high-level delegation to investigate and deliver a full report.