Afghan officials and local witnesses say a U.S. service member killed at least 15 Afghan civilians Sunday when he went on a shooting rampage in southern Kandahar province.

Officials say the service member walked off his base early Sunday and opened fire in multiples homes in the Panjwayi district.

U.S. officials say NATO has detained the service member and is investigating the incident.

Acting U.S. Ambassador to Afghanistan James Cunningham expressed the United States condolences to the victims' families in a video statement Sunday.

Cunningham says the U.S. “deplores” any such act by U.S. forces against innocent civilians, and ensured the service member will be brought to justice.

NATO's International Security Assistance Force deputy commander Lieutenant General Adrian Bradshaw expressed his “profound regrets and dismay” at the incident.

Bradshaw said in a statement that he cannot explain the motivation for such “callous acts” that have killed and injured “a number of civilians” in villages close to the base. He said the incident was in “no way” part of ISAF activities.

It remains unclear what might have motivated the service member's actions.

The shooting comes after weeks of tense relations between U.S. forces and their Afghan hosts, following the burning of Qurans at a U.S. military base.

Despite a U.S. apology for what it says was an accident, the burning of Qurans sparked violent protests and attacks across the country, killing dozens of people. There has also been a series of attacks by Afghan soldiers against foreign troops.