KABUL, Afghanistan – Afghan police killed four American soldiers coming to their aid after a checkpoint attack Sunday, the third assault by government forces or insurgents disguised in military uniforms in as many days.

The escalating violence – including a NATO airstrike that killed eight Afghan women and girls gathering firewood – is straining the military partnership between Kabul and NATO as the U.S. begins to withdraw thousands of troops sent three years ago to route the Taliban from southern strongholds.

The attacks drew unusually strong criticism Sunday from the U.S. military's top officer, Army Gen. Martin Dempsey, who called the problem of rogue Afghan soldiers and police turning their guns on allied troops "a very serious threat" to the war effort.