REPORTING FROM KABUL, AFGHANISTAN -- A lone American serviceman slipped away from his base in southern Afghanistan before dawn Sunday and went on a methodical house-to-house shooting rampage in a nearby village, killing 16 people, nearly all of them women and children, according to Afghan officials who visited the scene.

The NATO force confirmed that the assailant was in military custody, and that he had inflicted an unspecified number of casualties during the shooting rampage at about 3 a.m. Sunday. The U.S. Embassy called for calm and expressed deep condolences; the Taliban referred to the killings as an “act of genocide.”

The British Broadcasting Corp. reported that the shooter was a staff sergeant and a member of the U.S. special operations forces who had been involved in training the Afghan police. [Updated at 10:03 a.m.: The BBC later retracted the report.]

PHOTOS: Afghanistan shooting

The incident, potentially the worst atrocity of the 10-year war to be deliberately carried out by a single member of the Western military, represents a stunning setback to U.S.-Afghan relations, already shaken by last month’s burning of copies of the Koran at a U.S. military base north of Kabul.

Anti-U.S. sentiment flared into deadly riots after the Koran-burning at Bagram airfield came to light. American officials have said the action was a mistake and offered profuse apologies, but some Afghans, including lawmakers and senior clerics, brushed aside the apologies and called for harsh punishment of those involved.

The shooting early Sunday took place in Panjwayi district outside Kandahar city, in a village called Alkozai. U.S. military officials, speaking on condition of anonymity, said it was believed that the assailant had suffered a mental breakdown.

The NATO force issued a terse statement confirming casualties and promising a full investigation by U.S. and Afghan authorities. Later, the acting commander of the U.S.-led coalition, Lt. Gen. Adrian Bradshaw, expressed “deep regret and sorrow at this appalling incident.”

“I cannot explain the motivation behind such callous acts, but they were in no way part of authorized … military activity,” he said.

In the hours after the shooting rampage, casualty counts varied widely. By late afternoon, however, an official provincial delegation had arrived at the scene.

Haji Agha Lalai Dastgeeri, a member of that team, said the official tally was 16 dead. Nine of them were women, four were children and three were men, he said.

“I saw the dead bodies and visited the victims’ families,” he said soberly.

Earlier, Haji Mohammad Ehsan, the deputy head of Kandahar’s provincial council, had put the number of dead at 18. Javed Faisal, a spokesman for the Kandahar media center, said “up to 15” people had been killed, and several others wounded. The conflicting casualty counts could not immediately be reconciled.

The attacker’s motive was unknown, but relations between the U.S. military and ordinary Afghans have been highly fraught since February’s Koran-burning riots, which exacerbated longstanding tensions over civilian casualties and night raids led by U.S. special forces.

During more than a week of nationwide protests over the burning of the holy books that left at least 30 people dead, six U.S. service members were shot and killed by Afghan soldiers or, in the case of two of them, a worker at Afghanistan’s Interior Ministry. Two of the American troops who were killed were deployed in Kandahar province.

Kandahar, which is President Hamid Karzai’s home province, is also the birthplace and spiritual home of the Taliban. Panjwayi district was the scene of heavy fighting two years ago as U.S. forces made a major push to dislodge the insurgents, and parts of the district remain volatile.

The episode is certain to complicate U.S. dealings with Karzai, who has been resistant to American plans to try to inaugurate peace talks with the Taliban movement in the Gulf state of Qatar. The shooting comes days after an agreement to hand suspected insurgents in American custody over to the control of Afghan officials -- a process that is expected to take some months. Karzai had demanded an immediate handover of the main U.S. detention center.

Civilian casualties -- almost always accidentally inflicted when they come at the hands of the Western military -- have long been a sore point in the West’s dealings with Karzai. In a statement, the U.S. Embassy denounced “all violence against civilians,” and promised that the “individual or individuals responsible for this act will be identified and brought to justice.”

RELATED:

Afghan women fight it out in Herat

U.S., Afghans agree on prisoner handover

Pakistan's Iran pipeline plan further strains ties with U.S.

-- Laura King

Photo: Spectators look at the covered bodies of victims allegedly killed by a U.S. serviceman in Panjwayi district in Kandahar, Afghanistan. Credit: I. Sameem / EPA