U.S. and Afghan military officials say six American troops were killed Friday by Afghan government personnel.

In the first incident Friday morning, officials said an Afghan police commander shot and killed three special operations Marines in the Sangin district of Helmand province. Afghan authorities say the commander invited the Marines to a meal at his checkpoint in southern Afghanistan and then fled the scene after opening fire.

A Taliban spokesman said the attacker joined the insurgency after the shootings.

In the second attack, officials said that Friday night, an Afghan civilian employee working at a NATO base not far from the earlier shooting turned a gun on NATO members, killing three American troops.

The latest shooting was the fourth time this week that international forces have been fired on by their Afghan counterparts.

NATO troops killed an Afghan solider Thursday who tried to gun down coalition service members at a military base in eastern Afghanistan.

On Tuesday, NATO says two gunmen wearing Afghan National Army uniforms killed a coalition soldier in the east.

About 34 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 told reporters in Kabul on Saturday that the “isolated” incidents “don't reflect the overall situation” in Afghanistan and will have “no effect” on the security transition.