A fresh report published by the United Nations human rights office and its so-called Multidimensional Integrated Stabilization Mission in CAR (MINUSCA) detailed a massacre in Boyo village in the south of the country late last year.
Machete-wielding militia members killed at least 20 civilians in the week-long attack, while five women and girls were raped and 547 houses were burned and looted.
More than 1,000 villagers forced to flee, while hundreds of others were held for days in the village mosque as the militia members threatened to kill them, the report said.
The attack was one of the first instances when the government with private military companies "reportedly trained and armed locally recruited youth and created militias to enter villages under the control of armed groups", the UN rights office said in a statement.
The UN report said the Boyo attack appeared to have been aimed at punishing the Muslim community in the village, which was perceived as supportive of an armed group engaged in fighting the government, it said.
The report said most of the attackers were former members of the mainly Christian and animalist militia known as Anti-Balaka, but that dozens of local young recruits also took part, as well as members of private security companies and government forces.
Local recruits, trained and guided by the army and foreign private military contractors, used their knowledge of the area to reach the village and identify their victims, it added.
The attack, it said, confirms documented trends "where foreign private military contractors, operating under the direction or with the consent and acquiescence of the government, use proxies to perpetrate attacks on the civilian population."
One of the private companies in the CAR is the Russian-based "Wagner Group," accused of atrocities and human rights violations in a number of countries. Along with CAR, Wagner is also active in Mali, where it has supported the military government against all challengers.
The report concluded that the acts perpetrated in Boyo may constitute war crimes and crimes against humanity.
"I strongly condemn these horrific acts," UN rights chief Michelle Bachelet said in a statement.
"The government must put an end to all violations, whether by its forces, affiliated pro-Government militias, or foreign private military contractors, and hold all those directly and indirectly involved to account."
In their official response to the report, CAR authorities insisted the UN allegations were "not corroborated by the evidence".