The aid agency Caritas says more than 400 people have been killed by Ugandan rebels in the Democratic Republic of Congo in attacks since Christmas day.
Agency chief Dr Bruno Mitewo said they were hacked to death and forced into fires.
He told the BBC some 20,000 people have fled into the forests to escape from the Lord's Resistance Army, and a further 6000 have sought sanctuary around churches.
The rebels deny carrying out the attacks.
Forces from Uganda, South Sudan and DR Congo carried out a joint offensive against the rebels in mid-December after LRA leader Joseph Kony again refused to sign a peace deal and demanded that arrest warrants for him and his associates be dropped.
He is wanted by the International Criminal Court for war crimes and crimes against humanity.
News of the attacks began to emerge after the weekend when the Ugandan army accused the LRA of hacking to death 45 civilians in a Catholic church near Doruma.