Three Chadian soldiers embedded in the peacekeeping force were injured in a separate incident in Tessalit.
In a statement on Sunday, UN scribe António Guterres condemned both incidents as heinous and called for stiff measures against those responsible.
Mali had descended into instability in 2012 after militants occupied the north of the country, prompting a UN intervention force.
French troops stationed in the country have largely reversed their territorial gains but the militants still hold some parts of northern Mali.
The UN peacekeeping operation in the country is regarded as one of the most dangerous in the world with some 200 of its personnel killed since the mission began in 2013.