A man in an Afghan police uniform has gunned down three NATO personnel in the war-torn country's troubled south, the coalition said, the latest so-called "green on blue" attack.

The deaths take the toll this year in "green-on-blue" killings -- in which Afghan forces turn their weapons against their Western allies -- to at least 26, in a total of 18 such incidents.

In keeping with its usual policy, NATO's International Security Assistance Force  gave few details of the incident and did not reveal the nationality of the victims."

An individual wearing an Afghan national civil order police uniform turned his weapon against International Security Assistance Force service members in southern Afghanistan today, killing three service members," ISAF said in a statement.