A U.S. 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.

U.S. 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 U.S. 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 U.S.-led NATO forces in Afghanistan.

Anti-American sentiment had already been running high before news of the latest civilian casualties.

Anger gripped the country after U.S. soldiers burned a large number of copies of the Koran at a NATO base last month. NATO said it was a tragic blunder.

Thirty people were killed in protests and Afghan forces turned their weapons on U.S. soldiers, killing six.

(Writing by Michael Georgy; Editing by Daniel Magnowski)