Fifteen peacekeepers have been killed and 53 were wounded in an attack by rebels in the Democratic Republic of the Congo (DRC), a United Nations spokesman said.
Deputy spokesman Farhan Haq described it as a "huge" attack in North Kivu province on Thursday night and said that at least five Congolese soldiers had also been killed.
It is the worst violence against the mission in the Central African country in years.
"It's a very huge attack, certainly the worst in recent memory," Mr Haq said.
Mr Haq said the peacekeepers are mainly from the Tanzanian contingent and blamed the attack on rebels based in neighbouring Uganda.
UN Secretary-General Antonio Guterres called the attack "a war crime" and urged Congolese authorities to swiftly investigate. He expressed "outrage and utter heartbreak".
UN peacekeeping chief Jean-Pierre Lacroix said he is "outraged" by the attack and that medical evacuations are ongoing.
The base is approximately 27 miles from the town of Beni which has reportedly been repeatedly hit by rebels from the Allied Democratic Forces (ADF) rebel group.
Radio Okapi, which is linked to the peacekeeping mission known as MONUSCO, citing military forces said fighting at the base on Thursday lasted four hours.
It is home to the peacekeeping mission's rapid intervention force which has a rare mandate to go on the offensive.
There have been numerous conflicts in DRC since the UN mission's arrival in 1999 and nearly 300 peacekeepers have been killed since then, according to peacekeeping data.
In 2006, the UN mission helped carry out the country's first free and fair elections in 46 years, but since then the winner of that vote, President Joseph Kabila, has become further entrenched in his post.
Anger has grown as presidential elections originally set for late last year have been repeatedly delayed.
The nation has seen immeasurable cruelty and greed in recent history as a result of its vast mineral resources.
It suffered one of the most brutal colonial reigns ever known before undergoing decades of corrupt dictatorship and civil wars.
Human rights groups say the greatest threat to the region now comes from the ADF which, according to human rights group have been responsible for killing 1,000 in the last three years.