Two Afghanistan police officers have opened fire during a training session with Romanian Special Forces, killing two and wounding a third.
The two members of the local police unit then killed themselves, officials say.
A statement from the Romanian defence ministry said the incident took place while the men were training Afghan police in the southern city of Kandahar on Saturday.
So-called "green-on-blue" insider attacks by Afghan troops on international service members have occurred periodically over the years but have become much less frequent as a result of improved security measures.
The attack came as NATO commander General John Nicholson reviews plans to cut the number of US troops in Afghanistan by next year almost in half, a move that officials say would see the training mission severely reduced.
However, a statement from the NATO-led international coalition in Afghanistan said the shooting would not threaten its training and advisory mission with Afghan forces.
"We continue to train, advise and assist the ANDSF, and do not view this incident as representative of the positive relationship between our forces," it said.
Romania contributes almost 600 soldiers to the nearly 12,500-strong Resolute Support mission in Afghanistan.
Samim Khpalwak, a spokesperson for the Kandahar provincial governor, confirmed the shooting and said one Afghan policeman had been arrested and was being questioned.
Under current plans, U.S troop numbers are due to drop to from 9800 to 5500 by the start of 2017 but there has been growing speculation that Nicholson could recommend delaying the drawdown in order to keep the training mission going.