About a dozen U.N. peacekeepers battled some 50 unidentified attackers in the early hours of Wednesday at their base in Kirumba, in eastern Congo's troubled North Kivu province, where several armed groups are engaged in combat.
Six or seven other peacekeepers were injured in the attack, General Philippe Beni, MONUSCO chief of staff, told reporters.
"About 50 aggressors came out from the vegetation, they came discreetly at night and then hand-to-hand combat started between 10, 12 or so of our soldiers," he said.
The attack brings the number of peacekeepers killed in action to 34 since the mission began 10 years ago.
The world's largest U.N. peacekeeping mission has withdrawn 1,700 troops in recent months in response to calls from Congo's government to end the mission next year, but still supports operations against several armed groups in the east.
Roger Meece, the new head of mission, said those groups continued to be a "huge problem" menacing the population, and the U.N. would "continue and even enlarge our efforts" to weaken and eliminate them.
Meece said supporting a series of elections scheduled to start with a presidential vote on November 27 next year would be a priority for the mission, whose year-long mandate runs out next June.
"It's a priority for us to play our role in the elections and ensure free and transparent elections as in 2006," said Meece, formerly U.S. ambassador to Congo, adding that election registration had already started.
After 2006 elections, President Joseph Kabila's troops fought opposition candidate Jean-Pierre Bemba's forces in the streets of the capital in 2007.
Bemba is now awaiting the start of a war crimes trial at the International Criminal Court in the Hague, on charges relating to his role in fighting in Central African Republic, which neighbors Congo to the north.