UN police spokesman Neeraj Singh said two US police officers and a Jordanian were killed and 10 Americans and one Austrian wounded in the shooting on Saturday.
The lethal firefight between fellow members of the UN force was unprecedented in five years of peacekeeping in Kosovo, where police of 30 nations make up the international force of about 3500.
The 10-minute shootout took place in the UN compound in ethnically divided Mitrovica - a city that is more commonly the scene of clashes between Serbs and Albanians, in which UN police and NATO troops intervene to keep the peace.
The deputy head of the Serb hospital in Mitrovica, Milan Ivanovic, said one of the dead was an American woman, who was hit along with four female US police colleagues.
UN police sources said four Jordanian police officers had been arrested in connection with the shooting, but could give no further details on the cause.
A police source said it began with a row over Iraq. Singh said the UN was still investigating the possible motive.
The multinational UN police force is backed by the NATO-led KFOR military mission numbering about 20,000 troops.