The UN Security Council approved plans Tuesday to almost double the number of United Nations peacekeepers in South Sudan as soon as possible, with the aim of protecting civilians from worsening violence that has pushed the world's newest country to the verge of civil war.

The 15-member council unanimously authorized a request by UN Secretary General Ban Ki-moon to boost the strength of the UN mission in South Sudan to 12,500 troops and 1,323 police, up from its previous mandate of 7,000 troops and 900 police.

Mass grave discovered

Amid the eruption of violence along ethnic lines, UN investigators discovered a mass grave in a rebel-held city, the UN said Tuesday as a possible opening occurred for negotiations to avert civil war in the world's newest country.

The bodies were found in Bentiu: one grave with 14 bodies and a site nearby with 20 bodies, said UN 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 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.

20,000 seek safety at UN base

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

'I am extremely concerned that South Sudan risks spiralling into a disaster for both its own people and the region.'- Catherine Ashton, European Union

UN staff visited the mass grave in Bentiu on Monday. Originally the UN 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.

South Sudanese troops, meanwhile, are advancing on Bor in order to take it back from troops loyal to Machar, said military spokesman Col. Philip Aguer. The military said armed elements have entered a UN refugee camp in Bor, a frightening situation for the 17,000 civilians seeking refuge there.

Government troops will also soon advance on Bentiu, in oil-rich Unity state, he said.

The UN has staff in the country investigating mass killings, said Pillay. 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.

Death toll believed to surpass 1,000

The country's top UN humanitarian official 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. The official, Toby Lanzer, estimated that there are more than 100,000 internal refugees across the country seeking shelter from the violence.

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 spiralling into a disaster for both its own people and the region. Such a situation can, and must, be avoided," Ashton said, adding: "The people of South Sudan have suffered war for too long to be plunged once more into an avoidable conflict."

Hilde F. Johnson, head of the UN Mission in South Sudan called the scale of the crisis "unprecedented" for the mission, and called for more resources to help the about 45,000 people in UN camps across the country.

She also reiterated that mediation is key to finding a solution: "This is a political crisis and it can only be resolved through political means," she said. "There is no military solution to this conflict." She also called on all those in South Sudan to refrain from "any community motivated violence."

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.

With files from Reuters