Suspected rebels ambushed a Tanzanian unit of U.N. peacekeepers in eastern Congo on Tuesday, killing two and wounding 13, the spokesman for the secretary-general said in a statement. 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 late Tuesday.

The U.N. 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.

Mineral-rich eastern Congo has been plagued by a number of armed rebel groups, including the ADF, since the Rwandan genocide two decades ago. The peacekeeping mission, the largest for the U.N., includes a force intervention brigade with a unique mandate to take offensive military action against rebel groups.

The U.N. 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 U.N. envoy to Congo, Martin Kobler, has warned that making the $1.3 billion, 21,000-strong peacekeeping mission leave Congo too early would be disastrous.

Kobler tweeted Tuesday that he condemned the latest attack but that "MONUSCO will conduct robust offensive operations."

The U.N. 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."