Three UN police officers have died after a gunfight with their colleagues in the northern Kosovo town of Mitrovica, UN officials say.
It is not yet clear what provoked the gunfight between colleagues
The clash killed two Americans and one Jordanian and injured another 11 officers, most of them American.
Investigations are under way into what provoked the officers to turn their guns on each other.
Reports suggest the gunfight was triggered by an argument between the Americans and the Jordanian.
The UN uses more than 3,000 international police to maintain order between Kosovo's Serbs and Albanians.
But the UN has confirmed no Serbs or Albanians were involved in the latest shoot-out, which took place in the compound of a prison in Mitrovica.
'Drop the gun!'
One police force member said the Americans were leaving the prison in a bus, when the Jordanian officer opened fire on them.
An eyewitness quoted by the Associated Press news agency said he heard gunshots followed by the cries of American officers yelling, "Drop the gun! Drop the gun!"
The head of the UN's mission in Kosovo, Harri Holkeri, said he was "dismayed at the unfortunate death of dedicated professionals who have come such a great distance to help Kosovo on its road to the future".
The BBC's Nick Hawton in Pristina says the incident could put fresh pressure on the UN, which has also been accused of falling behind in its plan for political reform in the province.
The UN took charge of the Serbian province of Kosovo after a brief air war, in which Nato forces aimed to push back Serbian troops accused of war crimes against the ethnic Albanians.
The divided town of Mitrovica was the scene of deadly fighting between the two ethnic groups in March.