A NATO official said two U.S. soldiers were shot dead by an Afghan policeman in the western Farah Province on August 17.
NATO spokesman Jamie Graybeal confirmed the attack and said the Afghan policeman was also killed.
A NATO press release said the policeman appeared to have acted alone.
According to NATO figures, 39 foreign soldiers have been killed so far this year by Afghan workers or members of the Afghan security forces.
Based on reporting by AFP, dpa, and AP