Two NATO soldiers were killed in the south of Afghanistan and another injured when two attackers wearing the uniform of Afghan security forces opened fire on them, NATO reported.
Romanian defence ministry officials said those killed were members of the Romanian special forces.
The troops were shot while training police in the city of Kandahar.
There have been several incidents in recent years of Afghan security forces turning their arms on coalition troops.
However more recently so-called “green-on-blue” insider attacks – usually committed by Taliban members or supporters – have become much less frequent.
The attackers on Saturday were killed, a NATO statement said, and an investigation is under way.
One Afghan policeman is reported to have been been arrested and is being questioned.
The latest violence comes as NATO commander General John Nicholson reviews plans to reduce the number of US troops in Afghanistan by next year almost 50% in a move that officials say would adversely effect the training mission.
The NATO statement insisted however that the incident would not jeopardise its training and advisory mission with Afghan forces.
Romania contributes about 600 soldiers to the nearly 12,500-strong Resolute Support mission in Afghanistan.
US troop numbers are due to be reduced from 9,800 to 5,500 by 2017, but there is increasing speculation that Gen Nicholson may request a delay in the drawdown to keep the training mission running.