At least one Afghan police officer has turned his gun on Nato troops at a remote checkpoint in southern Afghanistan - killing four American service members before escaping, according to Afghan and international officials.
It was the latest in a string of insider attacks by Afghan forces against their international counterparts.
The killings threaten 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.
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 region. 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," Mr 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.
Afghan officials said the checkpoint, in Zabul province's Mizan district, first came under attack from insurgents sometime around midnight. American forces came to help the Afghan police respond to the attack, said Ghulam Gilani, the deputy police chief of the province.
It was not clear if some of the Afghan police turned on their American helpers in the middle of the battle with the insurgents, or afterwards, or were somehow forced into attacking the American troops by the insurgents, Mr Gilani said.
"The checkpoint was attacked last night. Then the police started fighting with the Americans. Whether they attacked the Americans willingly we don't know," Mr Gilani said.