NAIROBI, Kenya — Amid the eruption of violence along ethnic lines in South Sudan, U.N. investigators discovered a mass grave in a rebel-held city, the United Nations said Tuesday as a possible opening occurred for negotiations to avert civil war in the world’s newest country.

The government, meanwhile, announced that its military forces had taken back another key city, Bor, from the rebels who held it over the last week.

The bodies were found in Bentiu: one grave with 14 bodies and a site nearby with 20 bodies, said U.N. human rights office spokeswoman Ravina Shamdasani.

The government minister of information Michael Makuei Lueth said Bentiu is under the control of rebels loyal to the country’s former vice president, Riek Machar, indicating they were responsible for the killings.

The dead in Bentiu reportedly were ethnic Dinka who belonged to the Sudan People’s Liberation Army, said Shamdasani, referring to government military forces. South Sudan President Salva Kiir is Dinka while Machar is Nuer.

U.S. Secretary of State John Kerry spoke on the phone Tuesday with Machar, who said he told Kerry he is ready for talks with Kiir, likely to take place in Ethiopia.

“I will form a high-level delegation, to which I will give full power to negotiate an accord,” Machar told Radio France Internationale. “We want Salva Kiir to quit power. We want a democratic nation and free and fair elections.”

Violence began spreading across South Sudan after a fight among Kiir’s presidential guards late Dec. 15, pitting Nuer against Dinka.

Some 20,000 people seeking safety have crowded round the U.N. base in Juba, the capital, where at least two other mass graves are reported to have been found, U.N. human rights chief Navi Pillay said.

The U.N. Security Council is expected to vote later Tuesday on a resolution to temporarily increase the number of U.N. military personnel in South Sudan from 7,000 to 12,500, and an increase in the U.N.’s international police contingent from 900 to 1,323 following a call by Secretary-General Ban Ki-moon for more U.N. force.

U.N. staff visited the mass grave in Bentiu on Monday. Originally the U.N. said 75 bodies had been seen but later corrected that statement to 34 bodies seen and 75 people missing and feared dead.

The United States, Norway and Ethiopia are leading efforts to open peace talks on the 10-day-old crisis. Officials say Kiir and Machar have agreed to meet but specifics including the status of Machar’s imprisoned compatriots are holding up talks.