KANDAHAR, Afghanistan - Sixteen Afghan civilians, including nine children, were shot dead in what witnesses described as a nighttime massacre on Sunday near a US base in southern Afghanistan, and one US soldier was in custody.

While US officials rushed to draw a line between the rogue shooting and the ongoing efforts of a US force of around 90,000, the incident is sure to further inflame Afghan anger triggered when US soldiers burned copies of the Koran at a NATO base.

An elderly Afghan man sits next to the covered bodies of people who were killed by coalition forces in Kandahar province, March 11, 2012. [Photo/Agencies]

US officials said an American staff sergeant from a unit based in Washington state was in custody after the attack on villagers in three houses. Multiple civilians were also wounded, a spokesman for the NATO-led International Security Assistance Force (ISAF) coalition said.

President Barack Obama called his Afghan counterpart Hamid Karzai promising to establish the facts quickly and "to hold fully accountable anyone responsible."

There were conflicting reports of how many shooters were involved, with US officials asserting that a lone soldier was responsible, in contrast to witnesses' accounts that several US soldiers were present.

The incident was one of the worst of its kind since the US-led invasion of Afghanistan in 2001.

The US Embassy in Kabul said anti-US reprisals were possible following the killings, just as the Koran burning incident a few weeks earlier had touched off widespread anti-Western protests in which at least 30 people died.

Neighbors and relatives of the dead said they had seen a group of US soldiers arrive at their village in Kandahar's Panjwayi district at about 2 am, enter homes and open fire.

An Afghan man who said his children were killed in the shooting spree accused soldiers of later burning the bodies.

Obama said he was deeply saddened. "This incident is tragic and shocking and does not represent the exceptional character of our military and the respect that the United States has for the people of Afghanistan," Obama said in a statement.

US President Barack Obama talks on a phone with Afghanistan's President Hamid Karzai from his vehicle outside the Jane E. Lawton Community Center in Chevy Chase, Maryland, in this handout photograph taken and released on March 11, 2012. [Photo/Agencies]

"Intentional murders"

Afghan President 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.

Afghan officials also gave varying accounts of the number of shooters involved. Karzai's office released a statement quoting a villager as saying "American soldiers woke my family up and shot them in the face."

Minister of Border and Tribal Affairs Asadullah Khalid said a US soldier had burst into three homes near his base in the middle of the night, killing a total of 16 people including 11 people in the first house.

The ISAF spokesman said the US soldier "walked back to the base and turned himself into US forces this morning," adding there had been no military operations taking place in the area when the incident occurred.

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 has been a hive of insurgent activity in recent years.  

"I saw that all 11 of my relatives were killed, including my children and grandchildren," said a weeping Haji Samad, who said he had left his home a day earlier.