Afghan troops again turn guns on U.S.

An Afghan soldier stands guard at the scene of a suicide attack at the gate of an airport in Jalalabad, Nangarhar province east of Kabul, Afghanistan, Feb. 27, 2012.

(CBS News) KABUL - Two U.S. troops have were shot dead by three Afghan gunmen Thursday at their base in the southern Kandahar province, and two of the three shooters were Afghan National Army soldiers - another blow to a U.S. strategy pinned on direct partnership with Afghan forces.

The two Afghan troops who opened fire were killed by U.S. troops, and a third shooter, a teacher at the joint U.S.-Afghan base, was wounded.

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The protests which erupted after Korans were burned at the U.S. Bagram air base in Afghanistan may have died down, but targeted attacks on Americans are on the rise.

Right now, there is very little the U.S. military can do. Training and direct partnership with Afghan forces - eating together, sleeping in the same place and going on patrol together is a cornerstone of the U.S. military strategy in Afghanistan.

If U.S. troops doing the actual work have to worry not only about insurgents, but about potential enemies emerging without warning from within their own bases, that strategy is going to become increasingly difficult to follow.