A United Nations official says that armed gunmen fired on a U.N. vehicle in Mali's capital, killing one peacekeeper and injuring another.

The U.N. official said Tuesday an investigation is being opened into Monday night's ambush on the peacekeepers working for the U.N. mission in Mali. He said the attackers have not been identified. The official spoke on condition of anonymity because he is not permitted to speak to the press.

The attack comes after weeks of flaring violence between separatist rebels, the military and armed groups allied with the military, mostly in Mali's north. The main separatist rebel groups didn't sign a negotiated peace deal on May 15. Last week the coalition of separatist groups said it was holding 19 soldiers captured in the north.