The chief prosecutor of the International Criminal Court said Thursday that his office is investigating reports that at least 87 people were found in a mass grave in Sudan's West Darfur state.

"If this oft-repeated phrase of 'never again' is to mean anything, it must mean something here and now for the people of Darfur that have lived with this uncertainty and pain and scars of conflict for two decades," Prosecutor Karim Khan told the U.N. Security Council.

In 2005, the council referred the situation in Sudan's Darfur region to the Hague-based tribunal. An investigation was opened into the reported crimes of genocide, crimes against humanity and war crimes. Several individuals have been indicted by the court, including former Sudanese President Omar al-Bashir, who was ousted from power in 2019. The ICC has issued two arrest warrants for al-Bashir on charges including genocide and war crimes, but he is still at large.

Khan said the ICC has ongoing authority to investigate crimes committed in Darfur and is looking at violence committed there since fighting erupted on April 15 between the Sudanese army and the paramilitary Rapid Security Forces. There have been numerous reports of violence against civilians, especially in the West Darfur capital of El Geneina.

"The investigations that we are looking at encompass also many allegations in West Darfur -- looting, extrajudicial killings, burning of homes. And also allegations in North Darfur," he said.

The U.N. human rights office said earlier Thursday that the dead were found outside El Geneina and included members of the Masalit ethnic group. The bodies of seven women and seven children were among those found in the grave. Local residents said they were forced to bury the bodies June 20-21.

The U.N. said there was credible evidence that Sudan's Rapid Support Forces and an allied militia were responsible.

U.N. High Commissioner for Human Rights Volker Turk condemned the killings and called for a "prompt, thorough and independent investigation."

In an interview Thursday with VOA at the White House, officials said they were "deeply troubled" by the report.

"This is completely unacceptable, and the United States will be obviously taking this up with our allies and partners and with the U.N.," said John Kirby, director of strategic communications for the National Security Council.

"But we condemn it. It's completely horrendous and unacceptable, and the Rapid Support Forces, they need to be held to account for this."

Paris Huang, White House correspondent for VOA's Mandarin Service, contributed to this report.