By Carl Odera and Michelle Nichols
JUBA/UNITED NATIONS, Dec 24 (Reuters) - The U.N. Security Council approved plans on Tuesday to almost double the number of peacekeepers in South Sudan in a bid to protect civilians from violence as the discovery of a mass grave fueled fears of ethnic bloodletting in the world's newest state.
The 15-member council unanimously authorized a plan by U.N. Secretary-General Ban Ki-moon's to boost the strength of the force in South Sudan to 12,500 troops and 1,323 police, as some 45,000 civilians seek protection at U.N. bases.
Violence erupted in the capital Juba on Dec. 15 and quickly spread, dividing the land-locked country along ethnic lines of Nuer and Dinka.
South Sudan seceded from Sudan in 2011 under a peace agreement to end decades of war in what was Africa's biggest state.
"My cousin and nephew were both caught and executed. How can I leave this place?" asked Gatjang, a 29-year-old Nuer at a U.N. base in Juba that was crammed with thousands of civilians. "Even here. What if they sneak inside and attack us?"
Western powers and east African states, keen to prevent more chaos in a fragile region, have tried to mediate between President Salva Kiir, a Dinka, and rebel leader Riek Machar, a Nuer, who was vice president until Kiir sacked him in July.
The fighting is affecting oil production, which accounts for 98 percent of government revenue. Petroleum Minister Stephen Dhieu Dau told Reuters that output had fallen by 45,000 barrels per day to 200,000 bpd after Unity state oilfields shut down.
Dau said production in Upper Nile state, where most of South Sudan's oil is extracted, was safe and outside the reach of rebels.
Kiir said on Tuesday that government troops had retaken control of the Jonglei state capital Bor, a key town that last week fell to rebels loyal to Machar.
Most fighting has involved Dinka and Nuer factions of the Sudan People's Liberation Army, with militias and marauding youths also reported to be attacking rival ethnic groups. Kiir and Machar both have said the conflict is political, not tribal.
U.N. High Commissioner for Human Rights Navi Pillay said a mass grave believed to contain the bodies of 75 ethnic Dinka soldiers had been discovered in the rebel-held city of Bentiu, capital of Unity state.
RISING DEATH TOLL
The Juba government said it was not responsible for the mass grave in Bentiu, and messaged on Twitter: "Bentiu is currently under the control of the rebel leader Riek Machar - we have nothing to do with that area & the mass killings #SouthSudan."
Pillay said in a statement that there were also "reportedly at least two other mass graves in Juba."
She urged both sides to protect civilians, and said political and military leaders could be held to account for crimes. "Mass extrajudicial killings, the targeting of individuals on the basis of their ethnicity and arbitrary detentions have been documented in recent days," she said.
Hilde Johnson, the U.N. Special Representative of the Secretary-General, said a previous estimate of 500 dead, made on the third day of the 10-day-old conflict, would rise.
"It is likely that the numbers are much higher than initial calculations but we do not know," she told reporters in Juba.
The U.N. Office for the Coordination of Humanitarian Affairs said the crisis had displaced an estimated 81,000 people, though the real number was likely to be higher.
U.N. envoys on prevention of genocide and responsibility to protect, Adama Dieng and Jennifer Welsh, warned in a statement, "Targeted attacks against civilians and U.N. personnel, such as those that occurred in Juba and Jonglei, could constitute war crimes or crimes against humanity."
While Kiir and Machar have said they are open to talks, Machar said on Monday he would only negotiate if his detained political allies were released, a demand the government rejected.
"I am extremely concerned that South Sudan risks spiralling into a disaster for both its own people and the region," the European Union's foreign policy chief Catherine Ashton said on Tuesday. "Such a situation can, and must, be avoided."
Kiir said on Tuesday government troops were back in Bor after losing the town on Sunday.
"Forces loyal to the government have taken Bor and (are) now clearing whatever forces that are remaining there," he told reporters.
Troops loyal to Machar control most of Unity state, but officials have said government forces still hold the oil fields, which are some of the most heavily militarized areas in the country. (Additional reporting by Stephanie Nebehay in Geneva; writing by Drazen Jorgic; Editing by Alistair Lyon, David Evans, Toni Reinhold)