Kandhar: Western forces shot dead 16 civilians including nine children in southern Kandahar province on Sunday, Afghan officials said, in a rampage that witnesses said was carried out by American soldiers who were laughing and appeared drunk.
One Afghan father who said his children were killed in the shooting spree accused soldiers of later burning the bodies.
Witnesses told Reuters they saw a group of US soldiers arrive at their village in Kandahar's Panjwayi district at around 2 am, enter homes and open fire.
The incident, one of the worst of its kind since the US-led invasion of Afghanistan in 2001, is likely to deepen the divide between Washington and Kabul.
The US embassy in Kabul said an American soldier had been detained over the shooting. It added that anti-US reprisals were possible following the killings, which come just weeks after US soldiers burned copies of the Koran at a NATO base, triggering widespread anti-Western protests.
Afghan President Hamid Karzai condemned the rampage as "intentional murders" and demanded an explanation from the United States. His office said the dead included nine children and three women.
An Afghan minister earlier told Reuters that a lone US soldier had killed up to 16 people when he burst into homes in villages near his base in the middle of the night.
Panjwayi district is about 35 km (22 miles) west of the provincial capital Kandahar city. The district is considered the spiritual home of the Taliban and is believed to be a hive of insurgent activity.
Haji Samad said 11 of his relatives were killed in one house, including his children. Pictures showed blood-splattered walls where the children were killed.
"They (Americans) poured chemicals over their dead bodies and burned them," a weeping Samad told Reuters at the scene.
"I saw that all 11 of my relatives were killed, including my children and grandchildren," said Samad, who had left the home a day earlier.
Neighbors said they awoke to crackling gunfire from American soldiers, whom they described as laughing and drunk.
"They were all drunk and shooting all over the place," said neighbor Agha Lala, who visited one of the homes where the incident took place. "Their bodies were riddled with bullets."
A senior US defense official said Defense Secretary Leon Panetta "was deeply saddened to hear last night of this incident and is closely monitoring reports out of Afghanistan." The White House also expressed concern.
The Afghan Taliban would take revenge for the deaths, the group said in an e-mailed statement to media.
The US embassy in Kabul said an investigation was under way into Sunday's shooting and that "the individual or individuals responsible for this act will be identified and brought to justice".
The commander of NATO's International Security Assistance Force (ISAF) General John Allen said he was "shocked and saddened" by the shooting, and promised a rapid investigation.
The Minister of Border and Tribal Affairs Asadullah Khalid, who is investigating the incident, said the soldier entered three homes, killing 11 people in the first one.
"The defense minister ... is deeply shocked and saddened by the killings of 15 innocent civilians and the wounding of nine more at the hands of the coalition forces," the Defense Ministry in Kabul said in a statement.
Civilian casualties have been a major source of friction between Karzai's Western-backed government and US-led NATO forces in Afghanistan.
The shootings could intensify friction between Washington and Kabul as NATO prepares to hand over all security responsibilities to Afghans by the end of 2014, a process which has already started.
The Koran burning and the violence that followed, including a spate of deadly attacks against US soldiers, tested brittle ties between the governments of Karzai and President Barack Obama and underscored the challenges that the West faces even as it moves to withdraw.
All foreign combat troops will withdraw by end-2014 from a costly war that has become increasingly unpopular.