NATO officials in Afghanistan say a U.S. service member has been detained for his involvement in an incident in southern Kandahar province that resulted in Afghan casualties.

A provincial spokesman said Sunday the service member left his base and opened fire on civilians. It is not immediately clear how many people have been killed or wounded.

NATO says the incident is “deeply regrettable.” The international alliance says an investigation is under way and more information will be released “as appropriate.”

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. 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.