NATO says an Afghan police officer has killed two U.S. service members.

NATO said the service members were killed Friday morning in western Farah province. The alliance said the attacker was shot and killed.

Last week, six U.S. service members were killed by members of Afghan security forces or gunmen wearing their uniforms in two separate attacks.

More than 33 coalition service members have been killed this year in so-called “green-on-blue” attacks. “Green on blue” is a reference to the color of Afghan and NATO uniforms.

NATO spokesman Brigadier General Gunter Katz has said that the “isolated” incidents “don't reflect the overall situation” in Afghanistan and will have “no effect” on the security transition.