"Unless I receive appropriate responses from the government regarding a joint investigation by June 8, I will insist on the creation of an international investigative mechanism for the Kasais," said Zeid Ra'ad Al Hussein.
In March, the UN High Commissioner for Human Rights demanded a high-level investigation into abuses committed against civilians in the violence-wracked Democratic Republic of Congo following the discovery of dozens of mass graves.
The region has seen a major spike in violence since September when 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 more than 1.2 million from their homes, according to UN figures. Unconfirmed local statistics put the number of dead as high as 3,000.
In his opening address to the UN Human Rights Council on Tuesday, he warned that "the already dire situation in the Kasai province ... continues to deteriorate, spreading to other provinces and across the border with Angola."
His comments came after a coalition of 262 Congolese and nine international rights groups including Human Rights Watch and Amnesty International last week urged the Geneva-based rights council to set up the inquiry on the Kasai violence.
On Monday, Washington called for a UN-led probe into the murder of two UN experts who had been gathering evidence about 40 mass graves in the region.
Zeid also condemned the "difficulties in accessing the areas where violations and abuses are occurring" and said he would dispatch a team to the region next week "to meet with people fleeing attacks."