He added that local authorities had denied the UN access to information about what was happening in the region. The UN has said it has evidence that hundreds of villagers from the Luba and Lulua ethnic groups have been killed.
The UN Human Rights Council is likely to vote this week on whether to mandate an independent investigation into the violence following what the group’s commissioner described as horrific atrocities committed in Kasai province.
The Congolese authorities have said they would reject it.
More than a million people have been displaced in the region in the last year and aid workers say the humanitarian response on the ground has so far been inadequate.
Violence erupted in the once peaceful Kasai region last August, after the death of a local leader during fighting with security forces.