At least one Afghan police officer turned his gun on NATO troops at a remote checkpoint in southern Afghanistan before dawn Sunday, September 16, killing four American service members before escaping, according to Afghan and international officials.

It was the third attack by Afghan forces or insurgents disguised in military uniforms against international forces in as many days, killing eight troops in all.

Recent months have seen a string of such insider attacks by Afghan forces against their international counterparts. The killings have imperiled the military partnership between Kabul and NATO, a working relationship that is key to the handover of security responsibilities to Afghan forces as international troops draw down.

The most recent attacks also come amid an international uproar about an Internet video mocking the Prophet Muhammad that many fear could further aggravate Afghan-U.S. relations. The video has sparked protests throughout the Muslim world and the Afghan government blocked the YouTube site that hosts the video and its parent company, Google Inc., over the weekend in a move to prevent violent protests.

So far, protests in Afghanistan have remained peaceful.

Details of Sunday's attack were slow to come out because it took place in a remote area, said Jamie Graybeal, a spokesman for the international military in Afghanistan. He said the attacker was still at large but it was unclear if there were multiple assailants.

"The attack took place in the vicinity of an outpost in southern Afghanistan. It is my understanding that it was a checkpoint," Graybeal said. International forces often work with Afghan police to man checkpoints as part of the effort to train and mentor the Afghan forces so that they can eventually operate on their own. The goal is to turn over all security responsibility for the country to the Afghans by the end of 2014, though numbers of NATO forces have already been reduced in many areas, Associated Press reported.