Speaking in Geneva Zeid Ra'ad al-Hussein accused the DR Congo authorities of being complicit in ethnically inspired violence and said UN investigators had identified 42 mass graves.

The UN says its investigators found evidence that hundreds of villagers from the Luba and Lulua ethnic groups had been killed. Many of them, including young children, were shot dead or hacked or burned to death.

The investigators claim a new militia, the Bana Mura, is responsible for the atrocities.

The UN believes that the militia was set up specifically to support the Congolese authorities.

The UN also accuses the opposition Kamuina Nsapa militia of killing civilians thought to be loyal to the government, and of recruiting children as young as seven, many of whom are committing acts of violence themselves, apparently under the influence of drugs.

Violence erupted in Kasai last August, after the death of a local leader during fighting with security forces.Since then the UN estimates that more than a million civilians have been displaced.