Four peacekeepers were reported missing.
The ambush came a day after gunfire hit a helicopter carrying the mission's force commander in the same region, according to the statement released yesterday.
The UN said the Tanzanians came under fire by "suspected Allied Democratic Forces elements in Beni territory, North Kivu." It blamed "unidentified armed elements" for the helicopter incident.
"The secretary-general also condemns in the strongest terms the continuing atrocities perpetrated by the ADF against defenseless civilians in the Beni area," the statement said.
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The peacekeeping mission, the largest for the UN, includes a force intervention brigade with a unique mandate to take offensive military action against rebel groups.
The UN peacekeeping chief, Herve Ladsous, visited Congo late last month in an attempt to smooth relations with President Joseph Kabila, who wants the force, known as MONUSCO, cut significantly on the way to an eventual exit from the country.
The top UN envoy to Congo, Martin Kobler, has warned that making the USD 1.3 billion, 21,000-strong peacekeeping mission leave Congo too early would be disastrous.
The UN says that as of March 31, at least 86 peacekeepers in the mission had died, most by accident or illness. Just nine of those deaths were blamed on a "malicious act.