Zeid Ra'ad Al Hussein urged the UN Human Rights Council to establish an international investigation into serious abuses in the region, including summary executions, killings of children, recruitment of child soldiers and sexual violence.
"The scale and nature of these human rights violations and abuses, and the consistently inadequate responses of the domestic authorities, oblige us to call for an international investigation to complement national efforts," he said in a statement.
The UN children's fund UNICEF also issued a warning, saying the ongoing violence has severely disrupted the education of 150,000 children.
The flare-up in the Kasai region began last September, a month after government forces killed tribal chief and militia leader Kamwina Nsapu, who had rebelled against President Joseph Kabila.
The unrest has claimed more than 400 lives and forced some 1.3 million from their homes, according to the UN.
Unconfirmed local statistics put the number of dead as high as 3,000.
Zeid's office has meanwhile documented the existence of 42 mass graves, but his spokeswoman Ravina Shamdasani warned Friday the true number could be "much higher".
She said the rights office had information indicating that members of the Congolese armed forces, may have "dug many of the graves after clashing with presumed elements of the Kamuina Nsapu militia over the past several months."
Two UN rights experts were murdered in March while gathering evidence about mass graves in the region.
'Endemic impunity'
Zeid has since early May been urging the DRC government to "take a series of steps to (establish) a credible, transparent investigation, respecting international standards".
He had given Kinshasa a Thursday deadline only for Human Rights Minister Marie-Ange Mushobekwa to tell AFP that "one does not give ultimatums to a sovereign state."
Zeid acknowledged Friday that "it is the sovereign duty of the government of the DRC to carry out judicial investigations into human rights violations committed on its territory".
But he lamented that the government's "response to date falls short, in view of the gravity and widespread nature of the violations, and given the imperative need for justice for victims."
"The crimes committed in the Kasais appear to be of such gravity that they must be of concern to the international community as a whole, and in particular the Human Rights Council," he said, urging an end to "the endemic impunity in the DRC."
UNICEF's warning on the impact of violence on education was meanwhile stark.
"Attacks have damged 639 primary and secondary schools" in Grand Kasai, a UNICEF statement said, adding many schools were now serving as basic places of shelter for displaced persons.
"Lingering insecurity has created a culture of fear, leaving children and teachers unwilling or unable to return to the classroom," UNICEF added, saying fighting had disrupted the education of more than one in 10 children.
Last month, UNICEF also warned that the ongoing violence had left 400,000 children at risk of severe malnutrition.
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