A man wearing an Afghan police uniform shot dead two members of Afghanistan's NATO-led force in the south of the country on Tuesday, the force said.
An Afghan police official in the southern province of Helmand said a policeman had killed two British soldiers and efforts were underway to apprehend him.
NATO's International Security Assistance Force (ISAF) declined to say where the two members of the force killed in the attack were from.
So-called insider attacks on Western forces have undermined trust between coalition and Afghan forces as NATO prepares to withdraw most combat troops by the end of 2014.
At least 56 members of ISAF have been killed this year by Afghans wearing police or army uniforms.