The UN human rights office said on Thursday at least 87 people including women and children had been buried in a mass grave in Sudan's West Darfur, saying it had credible information they were killed by the country's Rapid Support Forces (RSF).
RSF officials denied any involvement, saying the paramilitary group was not a party to the conflict in West Darfur.
Ethnically motivated bloodshed has escalated in recent weeks in step with fighting between rival military factions that erupted in April and has brought the country to the brink of civil war.
Ethnic motivations for the attacks
In El Geneina, witnesses and rights groups have reported waves of attacks by the RSF and Arab militias against the non-Arab Masalit people, including shootings at close range.
"According to credible information gathered by the Office, those buried in the mass grave were killed by RSF and their allied militia around 13-21 June...," the UN statement said.
Local people were forced to dispose of the bodies including those of women and children in the shallow grave in an open area near the city between June 20-21, it added. Some of the people had died from untreated injuries, it said.
"I condemn in the strongest terms the killing of civilians and hors de combat individuals, and I am further appalled by the callous and disrespectful way the dead, along with their families and communities, were treated," said UN High Commissioner for Human Rights Volker Turk in the same statement, calling for an investigation.
An RSF senior official who declined to be identified said it "completely denies any connection to the events in West Darfur as we are not party to it, and we did not get involved in a conflict as the conflict is a tribal one."
Another RSF source said it was being accused due to political motivations from the Masalit and others. He reiterated that the group was ready to participate in an investigation and to hand over any of its forces found to have broken the law.
It was not possible to determine exactly what portion of the dead were Masalits, a UN spokesperson added.
The ethnic killings have raised fears of a repeat of the atrocities perpetrated in Darfur after 2003, when "Janjaweed" militias from which the RSF was formed helped the government crush a rebellion by mainly non-Arab groups in Darfur, killing some 300,000 people. Sudanese civilians have fled the area on foot, some having been killed or shot as they escaped.
"This report is a good first step, but more efforts are needed to uncover more violations," said Ibrahim, a refugee in neighbouring Chad, who asked to withhold his last name for fear of retribution.
Army spokesperson Brigadier General Nabil Abdullah told Reuters the incident "rises to the level of war crimes and these kinds of crimes should not pass without accountability."
"This rebel militia is not against the army but against the Sudanese citizen, and its project is a racist project and a project of ethnic cleansing," he said.