NAIROBI, Kenya (AP) — 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 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, 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 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.”

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.

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. 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 Security Council voted to temporarily increase the number of U.N. military personnel in South Sudan from 7,000 to 12,500, and the U.N.’s international police contingent from 900 to 1,323.

To reach the new levels, the resolution authorizes the temporary transfer of troops, police and equipment from the U.N. missions in Congo, Darfur, Abyei, Ivory Coast and Liberia. Ban recommended in a letter to the council on Monday that the peacekeeping mission be beefed up and asked for three attack helicopters, three utility helicopters, and a C-130 military transport plane as well as other needed equipment.

After the vote, the secretary-general said the resolution “will help boost security, reinforce peacekeeping bases and provide critical assets.” But he cautioned that strengthening the U.N. mission “will not happen overnight” — and even with additional manpower and equipment “we will not be able to protect every civilian in need in South Sudan.”

“The parties are responsible to end the conflict,” Ban stressed. The U.S. has moved extra forces to the region to provide embassy security and help with evacuations from South Sudan upon the State Department’s request. There are now about 150 troops in the region, with 10 aircraft, including Osprey helicopters and C-130 transport planes.

On Tuesday evening, U.S. Africa Command moved a platoon-sized Marine crisis response team and a KC-130J transport plane from the U.S. base in Djibouti to Entebbe, Uganda, to be able “to more quickly respond, if required, to help protect U.S. personnel and facilities” in neighboring South Sudan, said a Pentagon spokesman, Army Col. Steve Warren.

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. Adama Dieng, special adviser to the secretary-general on the prevention of genocide, and Jennifer Welsh, special adviser on the responsibility to protect, warned Tuesday that “targeted attacks against civilians and against United Nations personnel, such as those that have occurred in Juba and Jonglei, could constitute war crimes or crimes against humanity.”

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.

South Sudan’s U.N. Ambassador Francis Deng assured the Security Council after the vote that the government “is doing as much as it can, under very difficult circumstances, to restore calm and stability to the affected areas in the country.”

“South Sudanese do not want to fall back into the abyss of war from which they have suffered for over half a century,” he said. Deng repeated a statement from Kiir on Tuesday calling for an end to violence and ethnic targeting, and calling on Machar and the forces supporting him “to rise to the challenge of peace, unity and nation building.”

South Sudanese troops, meanwhile, advanced on Bor on Tuesday, said military spokesman Col. Philip Aguer. The government said on Twitter later that it was clearing out remaining rebel forces. The U.N. has staff in the country investigating mass killings, said Pillay, the U.N. rights chief. It is unclear who is responsible for the deaths, she said. Two other mass graves were reportedly found in Jebel-Kujur and Newside, near Eden, she said.

The country’s top U.N. humanitarian official, Toby Lanzer, said Monday that he believes the death toll from 10 days of violence has surpassed 1,000 but added that there are no firm counts. A top European Union official, Catherine Ashton, said political dialogue must include all groups, including those whose leaders are currently imprisoned. The use of force, she said, will achieve nothing.

“I am extremely concerned that South Sudan risks spiraling into a disaster for both its own people and the region. Such a situation can, and must, be avoided,” Ashton said. Hilde F. Johnson, head of the U.N. Mission in South Sudan, called the scale of the crisis “unprecedented” for the mission, and urged more resources to help people in U.N. camps across the country.

She also called on all those in South Sudan to refrain from “any community motivated violence.” The Security Council condemned “reported human rights violations by all parties, including armed groups and national security forces” and called for those responsible to be held accountable.

South Sudan, the world’s newest country, peacefully broke away from Sudan in 2011 following a 2005 peace deal. Before that, the south fought decades of war with Sudan. The country, one of the world’s least developed, still has pockets of rebel resistance and sees cyclical, tribal clashes that result in hundreds of deaths.