The United Nations says 13 new mass graves have been found in central Congo, bringing the total to 23 amid mounting violence in the area.

Jose Maria Aranaz, director of the U.N. human rights office in Kinshasa, said Tuesday that Congolese authorities would try to determine how many bodies were in the graves.

The mass graves are in a part of central Congo where at least 400 civilians have been killed since August amid a rebellion loyal to former traditional leader Kamwina Nsapu. The U.N. says at least 434,000 people are internally displaced.

Among the victims were two U.N. experts — American Michael Sharp and Swedish national Zaida Catalan — and their Congolese interpreter. Three others who were with them remain missing.