NATO says an Afghan police officer has killed two U.S. service members, in the latest so-called “green on blue” attack.

NATO said the incident took place early Friday in the western province of Farah. The coalition said the attacker was shot and killed.

Farah's police chief, Agha Noor Kemtoz, told reporters the Afghan soldier turned his gun on his U.S. counterparts during training. Afghan officials said the gunman had just recently joined the local police force.

In a second such attack on Friday, NATO said an Afghan soldier opened fire on foreign troops in the southern province of Kandahar, wounding two American troops and an Afghan service member. The soldier was shot and later died of his wounds.

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

At least 39 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 incidents “don't reflect the overall situation” in Afghanistan and will have “no effect” on the security transition.

But U.S. Defense Secretary Leon Panetta has acknowledged that the Taliban may be behind at least some of the attacks, telling reporters earlier this week that “the reality is the Taliban has not been able to regain any territory lost and so they're resorting to these kind of attacks to create havoc.”

Taliban leader Mullah Omar said members of the insurgent group have infiltrated what he called “the ranks of the enemy” as part of a plan developed last year.