Almost 600 people died in fighting in Sudan's Darfur region in May, making it the bloodiest month in the territory since 2008, according to a United Nations-African Union report.
The confidential document, which was leaked to the media on Monday, said 440 people had died in fighting between Darfur rebels and government forces, with another 126 killed in tribal violence.
The report, compiled by Unamid, the two organisations' joint peacekeeping force, said a further 31 people had been killed in other violence, including murder.
"The figure is very high. This was caused by tribal fighting and fighting with Jem [Justice and Equality Movement]," an unnamed official from Unamid said.
Renewed fighting between Jem, Darfur's main rebel group, and the government broke out in May after the fighters pulled out of peace talks.
The group has protested against the involvement of other rebels in the Qatari-brokered negotiations and has accused the government of breaking an earlier ceasefire.
Jem said on Monday it would continue to boycott the negotiations and was clashing "continuously" with Sudanese army forces in south and central Darfur.
Talks resumed in the Qatari capital, Doha, on Monday, but without the participation of Jem.
Negotiations were instead held between the Sudanese government and the Liberation and Justice Movement (LJM), a rebel group that is an alliance of splinter factions.
Amin Hasan Omar, the chief Sudanese negotiator, said Khartoum was "optimistic" about signing a final agreement with the LJM.
Ahmed bin Abdullah al-Mahmoud, a Qatari mediator and minister of state for foreign affairs, called on other groups to join in the negotiations.
"We hope the negotiations will yield the anticipated results, and [that] the rest of our brothers, without any exceptions, will join," Mahmoud said.
Jem signed signed a framework peace pact with the government in February, a move that was hailed by the international community as a major step toward bringing peace to Darfur.
But there was no final, comprehensive peace agreement by a March 15 deadline and Jem pulled out of the talks that same month, claiming ceasefire violations.
Darfur, a desert region the size of France, has been gripped by a civil war since 2003.
About 300,000 people have been killed in the fighting and another 2.7 million displaced, according to UN figures.
Khartoum says 10,000 people have died.