A US soldier has been detained in Afghanistan in connection with an incident involving civilian casualties in the southern Kandahar province, the Nato-led International Security Assistance Force (ISAF) said on Sunday.
"US Forces-Afghanistan, in cooperation with Afghan authorities, will investigate this incident," ISAF said in a statement.
"This is a deeply regrettable incident and we extend our thoughts and concerns to the families involved."
ISAF did not elaborate.
Kandahar Governor Tooryalai Wisa told Reuters a US soldier left his base in Kandahar on Saturday and opened fire on civilians. He said initial reports indicated up to 16 civilians were killed. That could not be immediately confirmed.
Civilian casualties have been a major source of friction between President Hamid Karzai's government and US-led Nato forces in Afghanistan.
Anti-American sentiment had already been running high before news of the latest civilian casualties.
Thirty people were killed in protests and Afghan forces turned their weapons on US soldiers, killing six.