"All armed groups must stop their activities and allow humanitarian access to the main victims of this tragedy, the civilian populations," envoy Martin Kobler said Wednesday.
The U.N. Office for the Coordination of Humanitarian Affairs reports more than 400,000 people have been displaced by recent fighting in southeastern Katanga province.
DRC conflict abated briefly after the March 23 Movement agreed last year to end its campaign against the government, though international aid groups say there are dozens of other armed groups fighting in the eastern DRC provinces.
Kobler said he was "extremely concerned" by the deteriorating situation in the region.
M23 waged mutiny against the DRC government in 2012. The United Nations estimates approximately 2.9 million people have been displaced by violence since the fighting began.
A special brigade within the U.N. peacekeeping mission in DRC has a mandate to use force in effort to bring stability to the country.