The UN on Friday detailed more than 250 "extrajudicial or targeted killings" of civilians in the Democratic Republic of Congo's Kasai region from mid-March to mid-June, counting dozens of children among those massacred.
The findings, based on interviews with 96 refugees from the conflict-hit Kasai who had fled to Angola, blamed state agents for the murders of seven children.
The refugees gave harrowing accounts of the violence in the central region, which the UN warned had taken on "an increasing and disturbing ethnic dimension."
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.
Aside from government troops, the UN blamed a reportedly state-backed militia called the Bana Mura as well as the anti-government Kamuina Nsapu militia for a range of atrocities.
"Survivors have spoken of hearing the screams of people being burned alive, of seeing loved ones chased and cut down, of themselves fleeing in terror", the United Nations human rights chief Zeid Ra'ad Al Hussein said in a statement.
A team of investigators has confirmed 251 executions between March 12 and June 19, the UN report said.
"These included 62 children, of which 30 were aged under eight".
Regarding the children murdered, the UN explained that seven were killed by members of the army (FARDC) or the national intelligence service, while six died at the hands of the rebel group Kamuina Nsapu.
The Bana Mura militia members were blamed for the deaths of 49 minors.
Scott Campbell, the head of the western and central Africa division at the rights office, said the new UN report was merely "a snapshot" of the wider conflict and atrocities had likely continued over the past six weeks.
The violence in the Kasai region "could amount to crimes against humanity", Campbell added, underscoring growing concern that the conflict was "tipping towards to ethnic cleansing".
The Kasai conflict erupted last September after the death in clashes of a tribal chieftain, known as the Kamwina Nsapu, who rebelled against the authority of President Joseph Kabila's regime in Kinshasa and its local representatives.
The killing sparked violence that has escalated, including gross alleged violations such as rapes, torture and the use of child soldiers.
The UN said the Bana Mura militia largely included members of the Tshokwe, Pende and Tetela ethnic groups, while the Luba and Lulua communities were seen as supporting the anti-government Kamuina Nsapu.
In less than a year, the violence has claimed more than 3,300 lives, according to a tally by the influential Roman Catholic Church, and displaced 1.4 million people.
Around 80 mass graves have been uncovered in the region.
The president's mandate expired last December but under a transition deal, he was allowed to remain in office until elections that are supposed to be held in late 2017.
Kabila has so far failed to set a date for the polls, heightening tensions across the country.
The UN rights chief has blasted Kabila's government for not mounting serious investigations into the Kasai crisis.
He successfully lobbied the Human Rights Council to set up an international investigation, although it is not clear if Congolese authorities will grant the probe access to the Kasai region.