U.N. spokesman Eduardo del Buey said the peacekeeping mission in the Democratic Republic of Congo had reports that fighting broke out between Congolese forces and M23 rebels near Goma, the capital of North Kivu province.
"The mission reports that initial skirmishes escalated to the use of heavy-caliber weapons, mortars and rocket launchers," he said in a statement.
M23 rebels seized Goma last year as part of their offensive against the government. M23 mutinied in 2012, complaining the government reneged on peace agreements. M23 leader Bosco Ntaganda surrendered to the International Criminal Court early this year to face war crimes charges.
Security has been volatile since the beginning of the year. The United Nations sanctioned a special intervention brigade with the peacekeeping mission that can conduct offensive operations to ensure the peace. Rwanda President Paul Kagame told the BBC this week he was concerned that U.N. forces in DRC weren't doing much for stability, however.
Del Buey said peacekeepers were concerned about the latest outbreak of violence in Goma.
"The mission is actively pursuing a diplomatic and political solution to contain and end the clashes," he said.