KABUL , Afghanistan Two members of the U.S.-led coalition were killed Saturday when two men wearing Afghan police uniforms opened fire on them at an undisclosed location in southern Afghanistan.

A spokesman for the International Security Assistance Force, the official name of the U.S.-led coalition, told McClatchy that one of the attackers was killed when coalition forces returned fire. A hunt is underway for the second attacker, who escaped.

ISAF refused to provide the nationality or gender of the dead soldiers, but said it had launched an investigation into the incident. It also declined to say where the attack had occurred, the other reports placed it in Helmand province, where U.S. troops have been battling the Taliban.

The attack is the latest in a series of so-called “green-on-blue” killings that have seen Afghan soldiers or policemen turn their weapons on their coalition colleagues. ISAF said last week that it had recorded 15 “green-on-blue” attacks so far this year which had resulted in 20 coalition deaths.

There were 21 such attacks in all of last year, killing 35 coalition members.

Service members from the U.S. and NATO countries are based with Afghan soldiers and policemen throughout Afghanistan as part of a major training and “mentoring” effort.

The program is supposed to prepare Afghan forces to take over responsibility for their nation’s security when the U.S.-led coalition withdraws most of its combat troops by the end of 2014.

However, the “green-on-blue” attacks have created doubt among coalition members about the trustworthiness of Afghanistan’s fledgling army and police force.

The Taliban have claimed responsibility for many of the incidents, saying its members have infiltrated the security forces.

But ISAF insists the attacks are generally isolated acts by Afghans angry at the way they have been treated by their foreign counterparts.