The UN experts said on Tuesday they had conveyed their concerns directly to Mali's government.
"We are disturbed by the apparent increased outsourcing of traditional military functions to the so-called Wagner Group in various military operations, also encompassing operations defined as counter-terrorism," they said.
"Mali must exert the utmost vigilance in prohibiting the direct participation in hostilities of all private individuals operating on its territory," they insisted.
"The use of mercenaries, mercenary-like actors and private security and military companies only exacerbates the cycle of violence and impunity prevailing in the country."
The UN experts said that since 2021 they had received "persistent and alarming accounts" of violations committed by Malian armed forces and their allies in the Mopti area and elsewhere.
Among the abuses were "horrific executions, mass graves, acts of torture, rape and sexual violence, pillaging, arbitrary detentions and enforced disappearances," they said.
The experts include members of the UN working group on the use of mercenaries, and the special rapporteurs on torture and on protecting rights while countering terrorism.
They said they were especially concerned about reports of mass executions last March at a village called Moura in central Mali.
Over several days, according to their information, "Malian forces, accompanied by military personnel believed to belong to the Wagner Group, executed several hundred people who had been rounded up."
Most of the victims were members of the Fulani, a minority, also called Peul, they said.
Human Rights Watch (HRW) in April last year had sounded the alarm over the bloodshed at Moura.
The group said 300 civilians had been killed in "the worst single atrocity" recorded in Mali's long-running conflict.
Wagner, an infamous Russian mercenary group founded in 2014, has been involved in conflicts in Africa, Latin America and the Middle East as well as Ukraine.
Earlier this month, the Uited States designated it a "transnational criminal organisation", putting it in league with Italian mafia groups and Japanese and Russian organised crime.