The U.S. military's top general says the problem of rogue Afghan soldiers and police turning their guns on American and allied troops is a "very serious threat" to the war effort.

Gen. Martin Dempsey says the Afghan government needs to take the problem as seriously as do U.S. and NATO commanders and officials.

The Joint Chiefs of Staff chairman says "you can't whitewash" the problem, and that it can't be fixed by just working harder.

Dempsey, who's traveling in Europe, tells the Pentagon's own news service, American Forces Press Service that "something has to change."

Four U.S. troops were killed in southern Afghanistan on Sunday in the latest insider attack by Afghan police. A day earlier, two British troops were killed in another such attack.

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