Suspected militiamen killed at least 14 U.N. peacekeepers and five Congolese soldiers in an attack on a United Nations base in eastern Congo on Thursday night, the U.N. mission said on Friday.
A further 53 U.N. 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 U.N. soldiers targeted in the attack were Tanzanian. A U.N. official confirmed that information.
Kambale said the attack occurred about 50 km (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.
Established in 2010, MONUSCO, the United Nations's 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 U.N. peacekeeping operations, Jean-Pierre Lacroix, wrote on Twitter.