NATO says an individual wearing an Afghan National Army uniform has opened fire on coalition forces in southern Afghanistan, killing two service members.

The alliance says the assailant was killed Monday when coalition forces returned fire. The attacker has not been identified.

The nationalities of the service members were not revealed.

Relations between Afghans and NATO troops have been strained by the shooting spree earlier this month allegedly by a U.S. Army soldier.

Staff Sergeant Robert Bales, an 11-year military veteran, has been charged with 17 counts of murder and six counts of attempted murder. He is alleged to have walked out of his southern Afghanistan military base under the cover of darkness March 11 and gunned down nine children and eight adults.