Two foreign soldiers and a security guard were killed in a shooting in southern Afghanistan on Saturday, sparking an investigation, the NATO-led force said.
NATO's International Security Assistance Force (ISAF) said it was probing the incident, but did not give any further details or reveal the nationalities of the victims.
Violence between coalition forces and security guards or Afghan forces is relatively rare, but not unheard of.
Last November, an Afghan border policeman in eastern Nangarhar province killed six US soldiers before being gunned down by others.
And in July, an Afghan soldier killed three British service members in southern Helmand province with gunfire and a rocket-propelled grenade before fleeing the base. The Taliban later claimed responsibility for both incidents.
ISAF also said on Saturday that a NATO soldier was killed by an improvised explosive device in the east of the country.
The deaths took to 91 the number of foreign soldiers killed so far in 2011, according to an AFP tally based on that kept by icasualties.org.
Around 140,000 foreign troops are in Afghanistan fighting a bloody insurgency waged by the Taliban since a US-led invasion toppled their regime in late 2001.
Last year was by far the deadliest yet for foreign troops, with more than 700 fatalities reported in the conflict. Southern and eastern Afghanistan have been worst affected by the violence.