The BBC's James Copnall explains the fighting gripping the world's newest state, South Sudan - in 60 seconds
The UN says it has reports of three mass graves in South Sudan, amid "palpable fear" among civilians they will be killed for their ethnicity.
There has been a week of fighting amid a power struggle between President Salva Kiir, a Dinka, and ex-deputy Riek Machar, of the Nuer.
The UN said 34 bodies had been found in Bentiu in the north, with two more sites reported in the capital, Juba.
One man in Juba said he was rounded up with 250 men and only 12 survived.
The fighting first erupted in the capital last week and has spread throughout South Sudan.
UN officials say at least 80,000 people have been displaced by the crisis - about half of them seeking shelter at UN bases - with fighting now having spread to half of the 10 states.
'Palpable fear'
UN High Commissioner for Human Rights Navi Pillay has urged both sides to do all they can to protect civilians.
She said in a statement: "Mass extrajudicial killings, the targeting of individuals on the basis of their ethnicity and arbitrary detentions have been documented in recent days."
UNHCR spokesperson Ravina Shamdasani said a UN official had seen 14 bodies in the grave in Bentiu and another 20 at a riverside nearby.
But she said 75 Sudan People's Liberation Army soldiers, all Dinka, were unaccounted for there and were feared dead.
Ms Shamdasani said: "As for the other two reported graves in Juba, we are still working to verify but it is very difficult, and there are reports that some bodies may have already been burned."
The two sites are at Jebel-Kujur and Newside, near Eden.
Ms Pillay added: "There is a palpable fear among civilians of both Dinka and Nuer backgrounds that they will be killed on the basis of their ethnicity."
BBC Africa analyst James Copnall says the fighting in South Sudan is not an exclusively ethnic conflict - it is a military combat propelled by political ambitions.
But he says South Sudan does have deep ethnic divisions and they are being exacerbated by the fighting.
Salva Kiir has agreed to talks "without preconditions"
Riek Machar says he has formed a delegation for talks in Ethiopia
President Kiir and Mr Machar have both said they are willing to talk to try to end the conflict.
However, Mr Machar has said his detained political allies must first be freed, while Mr Kiir says there should be no preconditions.
Mr Machar told Radio France Internationale he had spoken to US Secretary of State John Kerry on Tuesday. Mr Machar said he had formed a delegation for talks and that they would probably be held in Ethiopia.
He said: "We want a democratic nation. We want democratic free and fair elections. We want Salva Kiir to call it a day."
'Rounded up'
Rebels supporting Mr Machar have seized the major towns of Bor and Bentiu, the capital of the oil-producing Unity State.
Troops loyal to Mr Kiir are advancing on Bor. Army spokesman Col Philip Aguer said: "We expect that we [will] retake Bor within 72 hours."
Mr Kiir has accused Mr Machar, who he sacked in July, of plotting a coup. Mr Machar denies he is trying to seize power.
Hannah McNeish, a journalist in Juba, told the BBC she had interviewed a man called Simon, living at a UN camp, who said he had been shot four times but managed to survive a mass killing by hiding under dead bodies.
"He tells of being rounded up with about 250 other men, driven to a police station in one of Juba's busiest suburbs. He describes an ordeal whereby over the course of two days, forces outside the windows fired into this room, killing all but 12 men," she said.
The official death toll in the week of fighting stands at 500, but aid agencies say the true figure is likely to be much higher.
UN Secretary General Ban Ki-moon has called for an increase in the UN's peacekeepers from 6,800 to about 11,800. A Security Council vote on the resolution is expected later on Tuesday.
Sudan suffered a 22-year civil war that left more than a million people dead before the South became independent in 2011.
Fighting erupted in the South Sudan capital, Juba, in mid-December. It followed a power struggle between President Salva Kiir, a Dinka, and his Nuer ex-deputy Riek Machar. The fear is that the rivalry will spark a widespread ethnic conflict. According to OCHA, 81,000 people have been forced from their homes.
Have you been affected by the issues in this story? You can send us your comments and experience using the form below.
