An Afghan official says a U.S. service member walked off his base early Sunday in southern Afghanistan and began shooting Afghan civilians. It was not immediately clear how many people have been killed or wounded.

NATO says it has detained the service member and is investigating the incident in southern Kandahar province.

The international alliance says the shooting is “deeply regrettable,” and extended its “thoughts and concerns to the families involved.” NATO says more information well be released “as appropriate.”

There was no word on what might have motivated the service member's actions.

The shooting comes after weeks of tense relations between U.S. forces and and their Afghan hosts, following the burning of Qurans at a U.S. military base. Though U.S. officials apologized and said the burning was an accident, the incident sparked violent protests and attacks across the country, killing dozens of people.