KABUL, Afghanistan – An Afghan police officer killed at least 10 of his fellow officers on Saturday, a day after six U.S. service members were gunned down by their Afghan partners in summer violence that has both international and Afghan forces questioning who is friend or foe.

Attacks on foreign troops by Afghans working with the alliance are on the rise and, while cases of Afghan security forces killing within their own ranks are less frequent, together they show how battle lines have blurred in the decade-long war.

The assaults on international service members have stoked fear and mistrust of their Afghan allies, threatening to hamper the U.S.-led coalition's ongoing work to train and professionalize Afghan policemen and soldiers. The attacks also raise questions about the quality of the Afghan forces that have started taking charge of security in many areas of the country as U.S. and NATO combat troops move to withdraw by the end of 2014.