* Nearly 4 million displaced by South Sudan war
* U.N. refugee chief points finger at political leaders
* Tribal tensions, oil at root of conflict
By David Lewis
JUBA, June 18 (Reuters) - When South Sudan's civil war erupted in 2013, Nyayath Uluak was caught in crossfire in the northern town of Malakal and a bullet tore her leg apart. She survived, but the lower half of her limb didn't.
On leaving hospital, she found refuge with family in Yei, to the south, but then had to flee again as the war spread last year. Now, home is a camp outside the capital ringed by barbed wire and sandbagged positions manned by U.N. peacekeepers.
The elderly lady has little hope that South Sudan's warring leaders, President Salva Kiir, an ethnic Dinka, and his former deputy Riek Machar, a Nuer, will heal the personal and tribal animosity that split South Sudan just two years after it became the world's youngest nation.
"I don't think the leaders will resolve this any time soon," she said, sitting on the dirt floor of a shelter that was once used to teach children grammar and maths, but is now occupied by new arrivals. "I don't even know what they are fighting over."
The camp outside Juba hosts 30,000 people, 50 percent more than it was designed for.
The fugitives are part of the biggest movement of people in Africa since the 1994 Rwandan genocide - some two million within South Sudan and nearly the same again outside its borders.
U.N. experts say the conflict, which also has roots in control of South Sudan's oil wealth, amounts to ethnic cleansing and risks escalating to genocide.
"People told me stories of fear and violence," U.N. High Commissioner for Refugees Filippo Grandi said after walking though the tent city, where the stench of latrines hangs in the air and children play football on pitches of baked red earth.
The destitute camp-dwellers took advantage of Grandi's presence to list their daily hardships.
Kun Chuol, an education coordinator, complained of congestion and a lack of rations. Angelina Nyagak, a women's representative, said those who left the camp to try to top up rations face the risk of attack and rape by armed men.
In the last major bout of fighting, in July, 60 people in the camp were killed. Today, white armoured personnel carriers are parked around the perimeter. Grandi pledged to speak on their behalf in meetings with South Sudan's leaders.
"I can only speak to their sense of responsibility, hoping there is one," he said. "The flight of people is a clear testimony that you are not taking enough responsibility for your own citizens. People wouldn't go away if they weren't afraid."
After several failed peace efforts, South Sudan has launched its own national dialogue, while regional leaders have sought to revive internationally brokered talks.
But few in the camp hold out much hope while Riek Machar, who is being held in South Africa under virtual house arrest, remains outside the negotiations.
"It is about power. The problem came when Riek said he would stand in the election against Salva," said John Wiyual, a Nuer whose home is just 5 km (3 miles) away but says he fears setting foot outside the camp.
"They just need to bring those two people back together. There will be no peace in South Sudan if Riek is excluded."
Grandi said the failure of previous agreements, from those that ended decades of war with Khartoum and granted South Sudan independence to the more recent peace efforts, meant rebuilding trust would be tough.
"There will have to be a bloody good agreement to convince people to go back," Grandi said. "And it has to be sustainable. There is nothing worse than people who go back and have to flee again."
(Reporting by Ed Cropley; Editing by Mark Potter)