The U.S. military has suspended most joint field operations with Afghan forces because so many Americans are being killed by the men they are training.

Afghan government troops or police, supposed allies, have turned their guns on NATO forces 36 times this year, killing 51, most of them Americans, CBS reports. That is more attacks than the last two years combined.

The order issued Sunday by Lt. Gen. James Terry effectively suspends "until further notice" most operations U.S. and Afghan troops carry out side by side. At headquarters, Afghans and Americans will still work together, but in the field small unit operations putting Afghan soldiers alongside Americans -- the guts of the U.S. strategy to turn the fighting over to Afghans -- will be suspended unless an exception is granted by a commanding general.

The order was issued after a bloody weekend in which four American and two British troops were killed by so-called "insider attacks" -- Afghans turning their guns on their supposed allies.

In addition, two Marines were killed and eight fighter jets destroyed by enemy fighters who penetrated the heavily fortified Camp Bastion Friday. The Pentagon Monday identified them as Lt. Col. Christopher K. Raible, 40, of Huntingdon, Pa., and Sgt. Bradley W. Atwell, 27, of Kokomo, Ind. Raible was commander of the Harrier squadron that had six of its planes destroyed in the assault. 

This Is the second order that curbs contact between foreign troops and their Afghan partners, the Associated Press reports, undermining the mantra that both sides are fighting the Taliban "shoulder to shoulder." The directive could jeopardize the U.S.-led coalition's key goal to get Afghan forces ready to take over security from foreign forces by the end of 2014 - just 27 months from now.

Joint Chiefs Chairman Gen. Martin Dempsey called the surge in insider attacks "a very serious threat to the campaign," CBS reported. One U.S. official put it simply: "We have got to do a better job at protecting our troops."  

Meanwhile, a protest in Kabul over the film that mocks the Prophet Muhammad turned violent Monday, with hundreds of men torching tires, cars and shipping containers and lobbing rocks at a U.S. base on the edge of the capital.