UN human rights investigators have warned of ethnic cleansing in the Democratic Republic of Congo, documenting the recent killings of more than 250 people, including 62 children.
The investigators based their new report on interviews in June with 96 people who fled the country's Kasai provinces into neighbouring Angola over the three previous months.
The report decried alleged violence involving a recently formed militia, Bana Mura, backed by Congolese security officials.
The office provided photos of survivors with dismembered limbs and deep scars.
In a statement, the UN human rights chief described accounts of "the screams of people being burned alive" and others who were "cut down".
The report provides a snapshot of the violence that erupted in the once-calm region a year ago. The UN has estimated the existence of 80 mass graves there.
At least 1.3 million people have been internally displaced, and at least 40,000 have fled to Angola.
UN human rights chief Zeid Ra'ad al-Hussein urged the government to "act now to prevent such violence from tipping into wider ethnic cleansing".
His office called on militia groups to lay down their weapons.
Violence in the Kasai region by the Kamwina Nsapu militia began last August with the killing of a regional tribal leader who had defied the government of President Joseph Kabila.
Access for UN investigators to the region has been difficult, and security concerns soared after the murder of two UN experts in the Kasai region in March.
Based on the accounts from people who fled between March and June, the new report counted 251 killings, attributing 150 of them to the Bana Mura and another 79 to the Kamwina Nsapu.
Government forces were blamed for another 22.
The Catholic Church has estimated more than 3,300 people have died in the fighting.
The UN human rights office, which follows a strict methodology, has estimated the total death toll at more than 500.
The diamond-rich region has been a stronghold of opposition to Mr Kabila's government.
The Kamwina Nsapu, linked to the ethnic Luba and Lulua communities, have used children to attack security forces, the report said.
That militia group has also employed witchcraft, the rights office said, citing cases of "young girls drinking victims' blood as part of a magic ritual that was supposed to render the group invincible".
The Bana Mura, formed among the Tshokwe, Pende and Tetela ethnic groups, allegedly attacked Luba and Lulua inhabitants, "beheading, mutilating and shooting victims; in some cases burning them alive in their homes", according to the rights office.
Speaking to reporters in Geneva, Scott Campbell, the rights office's chief for central and west Africa, said: "There are no good guys and bad guys in this crisis. All sides need to be held to account."
The violence in the Kasai region comes on top of a broadly unstable situation in Congo, which has faced years of tensions and bloodshed in the east and where Mr Kabila's government has defied international calls for Congo to hold elections as required under its constitution.
The government says it needs more time.