Rebels killed at least 14 U.N. peacekeepers and wounded 53 others in Congo and U.N. Secretary-General Antonio Guterres said on Friday the attack that targeted troops from Tanzania was the worst in recent history.
Guterres said the raid in the east of the Democratic Republic of Congo constituted a war crime and he called on Congolese authorities to investigate and "swiftly bring the perpetrators to justice".
"I want to express my outrage and utter heartbreak at last night's attack," Guterres told reporters at U.N. headquarters in New York.
Three other peacekeepers are missing after a three-hour firefight that broke out at dusk on Thursday evening, said Ian Sinclair, the director of the U.N. Operations and Crisis Centre.
Suspected militants from the Allied Democratic Forces (ADF) staged the assault on a base for Congo's U.N. mission (MONUSCO) in the town of Semuliki, the mission said.
The ADF is an Islamist rebel group from across the border in Uganda, who has been active in the area. MONUSCO said it was coordinating a joint response with the Congolese army and evacuating wounded from the base in North Kivu's Beni territory.
Five Congolese soldiers were also killed in the raid, MONUSCO said in a statement.
Rival militia groups control parts 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 of whom died from hunger and disease.
The area has been the scene of repeated massacres and at least 26 people died in an ambush in October.
The government and U.N. mission have blamed almost all the violence on the ADF but U.N. experts and independent analysts say other militia and elements of Congo's army have also been involved.
Increased militia activity in the east and centre of the country and a series of prison breaks have fuelled mounting insecurity in Congo this year amid political tensions linked to President Joseph Kabila's refusal to step down when his mandate expired last December.
An election to replace Kabila, who has ruled Congo since his father's assassination in 2001, has been repeatedly delayed and is now scheduled for December 2018.
Established in 2010, MONUSCO is the United Nations' largest peacekeeping mission and had recorded 93 fatalities of military, police and civilian personnel.
The death toll for the attack varied. A spokesperson for Congo's army, Mak Hazukay, said only one Congolese soldier was missing after the fighting and one had been injured, adding that 72 militants had been killed.
Guterres said most of the peacekeepers were from Tanzania. Tanzania's defence minister declined to comment.