At least 14 UN peacekeepers have been killed and 53 wounded in an attack in the east of the Democratic Republic of Congo.
The UN's Monusco mission said the peacekeepers were attacked by suspected rebels of the Allied Democratic Forces (ADF) in North Kivu province.
Five soldiers with the DR Congo's armed forces (FARDC) were also killed.
UN Secretary-General Antonio Guterres described it as the worst attack on UN peacekeepers in recent history.
He sent his condolences to Tanzania, their home country.
Eastern DR Congo has suffered years of instability with rival groups fighting for control of territory.
A Monusco statement said rebels had launched an attack against an operating base at Semuliki in Beni territory on Thursday evening.
"This resulted in protracted fighting between suspected ADF elements and Monusco and FARDC Forces," it added.
The head of Monusco, Maman Sidikou, said: "I condemn in the strongest terms this deadly attack on United Nations peacekeepers and the FARDC. Monusco will take all actions to ensure that the perpetrators are held accountable and brought to justice."
Writing on Twitter, Jean-Pierre Lacroix, Under-Secretary-General for Peacekeeping, said reinforcements had been sent to the scene and medical evacuations were under way.
Mr Guterres expressed "outrage and utter heartbreak" at the attack, and sent his condolences to families of the victims.
Suspected ADF rebels killed two peacekeepers in October, a BBC correspondent in Kinshasa says.
Monusco is the UN's largest and most expensive peacekeeping operation and has been in operation since 2010.
However, earlier this year the UN revealed a plan to cut the number of peacekeepers there from about 19,000 to 3,000.
Several UN member states have signalled a desire to cut spending on peacekeeping.
Monusco has also faced violent demonstrations by civilians, who accuse it of being ineffective.
Several armed militias are fighting for control of mineral-rich lands in North Kivu and often clash with Congolese troops and UN peacekeepers.
Earlier this week, aid agencies said that conflict had forced 1.7 million people in DR Congo to flee their homes this year.
The Norwegian Refugee Council's DR Congo director, Ulrika Blom, described the situation as "a mega-crisis".
"The scale of people fleeing violence is off the charts, outpacing Syria, Yemen and Iraq," she said.