The latest incident in the Helmand province marked the third so-called "insider" attack in as many days.
NATO's International Security Assistance Force spokesman Jamie Graybeal gave only sketchy details, saying the troops were killed "following an insider attack involving suspected Afghan police," CNN reported.
The location within Helmand province and nationality of the soldiers weren't disclosed.
Insurgents are increasingly staging attacks while wearing Afghan police or military uniforms. As a result, NATO has stepped up background checks on existing and new recruits, although it isn't clear if the majority of attacks are actually Afghan security officers or insurgents wearing uniforms purchased on the black market, The Guardian newspaper in England said.
Saturday, two British soldiers were shot and killed by a man wearing an Afghan police uniform who opened fire at a checkpoint in Helmand province, the newspaper said.
A major insider attack Friday on adjoining British and U.S. military bases in Helmand killed two U.S. Marines and 14 insurgents, all of them wearing U.S. Army uniforms. One wounded insurgent was taken into custody, NATO said.
The mortar and small arms attack caused heavy damage to the British Camp Bastion airbase. The coalition said six AV-8B Harrier jets were destroyed and two others were damaged, while six hangers were damaged.