At least one Afghan police officer turned his gun on NATO troops at a checkpoint in southern Afghanistan before dawn today, killing four international service members before escaping, a spokesman for the international military coalition said.

It was the latest in a string of insider attacks by Afghan forces against their international counterparts that are threatening the military partnership between Kabul and NATO, a partnership that is key to the handover of security responsibilities to Afghan forces as international troops draw down. The day before, a gunman in the uniform of a government-backed militia force shot dead two British soldiers in the south.

Jamie Graybeal, a spokesman for the international military in Afghanistan, said details of the attack were slow to come out because it took place in a remote part of southern 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 some areas.

The coalition said in a statement that they are investigating what happened. The statement confirmed the deaths but did not provide details or nationalities. NATO typically waits for the nations contributing troops to identify their own dead.

There were also international troops wounded, Graybeal said. He said that early reports showed two were hurt and that they were receiving treatment. He did not say how serious the injuries were.

So far this year, 51 international service members have died at the hands of Afghan soldiers or policemen or insurgents wearing their uniforms. At least 12 such attacks came in August alone, leaving 15 dead.