Rebels have killed 14 peacekeepers and injured more than 40 in an attack on a United Nations base in east Congo.
At least five Congolese soldiers were also killed in the attack in North Kivu province in the worst attack "in recent memory", deputy UN spokesperson Farhan Haq said.
The peacekeepers were mainly from the Tanzanian contingent, he added.
The peacekeeping base is located about 45km (27 miles) from the town of Beni, which has been repeatedly hit by rebels from the Allied Democratic Forces rebel group.
Jean-Pierre Lacroix, UN under-secretary-general for peacekeeping operations, said he was "outraged" by the attack on Thursday evening. He said on Twitter that medical evacuations were ongoing from the scene, but did not identify the attackers.
The UN peacekeeping mission in the Democratic Republic of Congo is the largest and most expensive in the world and is aimed at calming a number of armed groups in the vast, mineral-rich Central African nation.
Radio Okapi, which is linked to the peacekeeping mission known as MONUSCO, reported that peacekeepers on Thursday repelled an attack by fighters with the Allied Democratic Forces (ADF) rebel group on a UN base in the Beni area. The base is home to the peacekeeping mission's rapid intervention force, which has a rare mandate to go on the offensive.
The radio station, citing military sources, said fighting lasted four hours. It reported that Congolese forces did not intervene because the closest ones were several miles away.
Congo, a country the size of Western Europe, has seen immeasurable cruelty and greed as a result of its vast mineral resources.
The nation suffered through one of the most brutal colonial reigns ever known before undergoing decades of corrupt dictatorship. Back-to-back civil wars later drew in a number of neighbouring countries.
The conflicts have been numerous since the UN mission's arrival in 1999. Many rebel groups have come and gone, at times invading the regional capital, Goma. One of the greatest threats in the region now comes from the ADF. Human rights groups say its fighters have killed about 1,000 people in the past three years.
The UN mission in 2006 helped carry out Congo'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.
Additional reporting by AP
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