NAIROBI, Kenya - U.N. investigators discovered a mass grave in a rebel-held city in South Sudan, the United Nations said Tuesday, as a possible opening occurred for negotiations to avert civil war in the world's newest country where ethnic violence has erupted.

In New York, the U.N. Security Council voted unanimously to beef up its peacekeeping force in South Sudan. It condemned targeted violence against civilians and ethnic communities and called for "an immediate cessation of hostilities and the immediate opening of a dialogue."

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 the town of Bentiu in oil-rich Unity state: 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 was 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, the country's largest ethnic group, while Machar is Nuer, the second-largest ethnic group.

U.S. Secretary of State John Kerry spoke on the phone Tuesday with Machar, who said he told Kerry he was 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."

U.N. Secretary-General Ban Ki-moon, reiterating his call for Kiir and opposition leaders to end the crisis, said: "Whatever the differences, nothing can justify the violence that has engulfed their young nation."

"There is no military solution to this crisis," Ban stressed. "This is a political crisis which requires a peaceful political solution."

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

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

The U.N. humanitarian office said 45,000 people have taken refuge in and around U.N. bases in the country, and 81,000 people have fled their homes as a result of the fighting.

The U.S. Africa Command moved about 50 Marines to neighboring Uganda, better positioning them to take action to protect U.S. facilities and personnel, if needed. The Marines were part of a 150-strong contingent deployed Monday from their base in Spain, along with transport and refueling aircraft, to Djibouti, on the Horn of Africa.

This article contains information from the Washington Post.