A peacekeeper has shot dead two colleagues in what has been called a "settling of scores", the UN peacekeeping mission in Mali said.
The suspect was immediately detained after the shooting at the UN base in Kidal in northern Mali, a statement from the mission said, and an investigation is under way.
The Chadian peacekeeper shot a commander and doctor, a peacekeeping official said. It is not clear what caused the shooting.
Operations at the base have returned to normal, mission spokesman Olivier Salgado said.
The UN mission in Mali has been one of the world body's deadliest peacekeeping missions.