Suspected militiamen killed at least 14 peacekeepers and five Congolese soldiers in an attack on a United Nations base in eastern Congo on Thursday night, the UN mission said on Friday.
A further 53 UN troops were wounded, it said.
The mission, known as Monusco, said it was coordinating a joint response with the Congolese army as well as medical evacuations of the wounded from the base in North Kivu’s Beni territory.
Rival militia groups still control swathes of mineral-rich eastern Congo nearly a decade and a half after the official end of a 1998-2003 war that killed millions of people, most from hunger and disease.
Gilbert Kambale, the president of an activist group in Beni, said the UN soldiers targeted in the attack were Tanzanian.
Kambale said the attack occurred about 50km (30 miles) northeast of Beni city on the road that leads to the Uganda border, near where militants killed at least 26 people in an ambush in October.
The UN peacekeeping mission is believed to have been targeted by Ugandan Muslim rebel group ADF, one of several armed groups active in the North Kivu region, according to Congolese military sources.
In a statement, UN Secretary General Antonio Guterres put the death toll at "at least 12" Tanzanian peacekeepers.
Guterres said it was the worst attack on UN peacekeepers in the organisation's recent history.
"I condemn this attack unequivocally. These deliberate attacks against UN peacekeepers are unacceptable and constitute a war crime," Guterres said.
"I call on the DRC authorities to investigate this incident and swiftly bring the perpetrators to justice. There must be no impunity for such assaults, here or anywhere else."
Established in 2010, Monusco, the United Nations’ largest peacekeeping mission, has recorded 93 fatalities of military, police and civilian personnel.
“Our thoughts and prayers with families and our colleagues in Monusco. Reinforcements are on scene and medical evacuations by mission ongoing,” the head of UN peacekeeping operations, Jean-Pierre Lacroix, wrote on Twitter.
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-Additional reporting by AFP