Concerns over genocide in Sudan were raised after the U.N. human rights office reported Thursday that at least 87 people, including ethnic Masalits, were buried in what it described as a mass grave in the country's West Darfur.

The U.N. body added that it had credible information that the Rapid Support Forces (RSF) were responsible for the deaths.

Ethnically motivated bloodshed has escalated in recent weeks in step with fighting between rival military factions that erupted in April.

The crisis has brought the country to the brink of civil war. 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.

Local people were forced to dispose of the bodies including those of women and children in an open area near the city between June 20-21, the U.N. statement said. 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 U.N. High Commissioner for Human Rights Volker Türk in the same statement.

He called for a prompt and thorough investigation.

An RSF spokesperson was not immediately available for comment.

It was not possible to determine exactly what portion of the dead were Masalits, a U.N. 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, allegedly killing some 300,000 people.

Army spokesperson Brig. Gen. 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.

Deaths in Omdurman

Earlier Wednesday the Sudanese Health Ministry reported at least 34 Sudanese civilian deaths in indiscriminate shelling of a marketplace in Omdurman city, west of the capital Khartoum.

A number of children were among the victim, the ministry said in a brief statement.

According to eyewitnesses, the attack occurred at 8:00 a.m. local time (0600GMT) on Tuesday and lasted over three hours.

Most of the casualties were traders and transport vehicle owners, they said.

The party behind the attack was not yet clear.

There was no comment yet from the Sudanese army or the RSF paramilitary group on the report.

Sudan has been ravaged by clashes between the army and the RSF since April, in a conflict that killed nearly 3,000 civilians and injured thousands, according to local medics.

Several cease-fire agreements brokered by Saudi and U.S. mediators between the warring rivals had failed to end violence in the country.

The International Organization for Migration (IOM) estimates that nearly 3 million people have been displaced by the current conflict in Sudan.

Last week, U.N. Secretary-General Antonio Guterres warned that the ongoing conflict in Sudan may lead to a full-scale civil war.