Word of possible massacres around Yumbi first surfaced a few weeks ago, when the United Nations’ human rights office said it had reports from “credible sources” of horrific violence between Dec. 16 and Dec. 18 that had killed 890 people. Michelle Bachelet, the top human rights official at the United Nations, said it was “crucial that this shocking violence be promptly, thoroughly investigated and the perpetrators be brought to justice.”

A fact-finding team sent to Yumbi and the nearby town of Bongende by the United Nations Organization Stabilization Mission in Congo, known as Monusco, found the burial sites, a spokeswoman, Florence Marchal, said Tuesday in a telephone interview.

In Yumbi, she said, “if you walk along the main road you see the graves.”

Ms. Marchal said Monusco had yet to establish an exact death toll. But confirming her remarks quoted in a Reuters report from Kinshasa, she said that “several hundred people, including women and many children, were killed in unbearable circumstances.”

Abdoul Aziz Thioye, the director of the United Nations Joint Human Rights Office in Kinshasa, said the graves had been dug by local Red Cross workers and family members who initially fled the violence and had returned to bury the dead.

“Very often when you talk about mass graves, everyone has in mind people who have been summarily executed and thrown into a hole, which was not the case here,” he said in a telephone interview.