Roger Meece, director of the U.N. Stabilization Mission in DRC, said via video conference peacekeepers were responding to violence instigated by an April mutiny led by rebel Gen. Bosco Ntaganda.
The International Criminal Court has sought the arrest of Ntaganda since 2006. Human Rights Watch said he recruited at least 140 boys and young men, of whom about 10 percent were under the age of 15, into his forces since he rebelled in mid-April.
Some of Ntaganda's associates wanted by the ICC are tied to the Democratic Forces for the Liberation of Rwanda, a militant group linked to the Rwandan genocide in 1994.
Meece said peacekeepers were working to protect civilians from fighting between Ntaganda's forces and those from the DRC military.
The fighting, he added in his statement, has led to the "significant" displacement of civilians.
The United Nations estimates around 30,000 Congolese people have fled the country because of recent fighting.
ICC Chief Prosecutor Luis Moreno-Ocampo last week sought to add war crimes and crimes against humanity charges to the arrest warrant for Ntaganda.