Congolese security forces and a militia fighting them have killed at least 3,383 people in the central Kasai region since October, the Catholic Church says.
Church officials, citing their own sources in the remote territory bordering Angola, said the army had destroyed 10 villages as it sought to stamp out an insurrection.
They also accused the Kamuina Nsapu militia of killing hundreds of people, destroying four villages and attacking church property in a campaign to drive out central government troops.
No one was immediately available to comment from the militia or Democratic Republic of Congo's army, which has dismissed accusations of excessive force in the past.
The clashes have triggered fears of a wider conflict in the central African giant, a tinderbox of ethnic rivalry and competing claims over mineral resources. Wars at the turn of the century killed millions and sucked in neighbouring countries.
The church's report will carry considerable weight in a country where about 40 per cent of the population identifies as Catholic.
Fighting surged in Kasai in August when the army killed a chief who had been calling for central government forces to quit the region, saying it should be left to local leaders.
The UN Human Rights Council in Geneva is due to decide this week whether to authorise an investigation into the Kasai violence. UN investigators say they have discovered 42 mass graves.
Congo's government opposes an international investigation, saying that would violate its sovereignty.
The United Nations says more than 1.3 million people have fled the fighting.
