KABUL, Afghanistan (AP) — 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.