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 Associated Press reports the two service members were British soldiers. NATO has not formally released information about the nationalities of the dead service members.

Afghan officials say the incident happened at the NATO base in Lashkar Gah, the main town in Helmand province.

U.S. officials say there have been at least 42 attacks by Afghans on NATO colleagues in Afghanistan since 2007.

The attack came two weeks after a U.S. Army soldier allegedly went on a shooting spree in Afghanistan, straining relations between Afghans and NATO troops.

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.