This is the first time the United Nations has directly suggested that government forces dug the mass graves.
More than 250 people, including 62 children, were killed in the Democratic Republic of Congo from mid-March to mid-June, UN human rights investigators said in a new report
At least 17 cases were of mutilations and other injuries, nine victims of abductions, four victims of rape and one victim of arbitrary arrest.
The OHCHR also received reports of incidents of widespread looting and destruction or burning of property and parts of villages.
Based on accounts from people who fled between March and June, the 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 Bana Mura militia were responsible for 171 victims [150 killed], the Kamuina Nsapu militia for 86 victims [79 killed] and FARDC soldiers for 25 victims [22 killed],” the report said.
UN human rights chief Zeid Ra’ad al-Hussein urged Congo’s government to “act now to prevent such violence from tipping into wider ethnic cleansing”.
Hussein described accounts of “the screams of people being burned alive” and others who were “cut down”.
Victims recounted mutilations, including of a seven-year-old boy whose fingers were cut off, and an attack on a hospital in the village of Cinq where 90 people were killed, some because they were too injured to escape a raging fire.
Violence in the Kasai region by the Kamwina Nsapu fighters erupted last August with the killing of a regional tribal leader who had defied the government of President Joseph Kabila.
The violence in the once-calm 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 Kabila’s government has defied international calls for Congo to hold elections as required under its constitution.
The Catholic church has estimated that more than 3,300 people have died in the fighting since the tribal leader was killed in a military operation a year ago.
Around 80 mass graves have been uncovered in the region.