UN sends more peacekeepers to South Sudan after thousands killed "The group of 150 people which is now in Djibouti has been sent to be better positioned quickly if the security situation in South Sudan demands it," an Africom spokesman told The Telegraph. They are primarily intended to assist to protect US citizens or property, and there I'm talking about the Embassy, should the State Department or any other arm of government request it. In his strongest statement yet on the 10-day-old crisis, U.N. Secretary-General Ban Ki-moon demanded an end to the violence and said crimes against humanity may already have taken place. "Those responsible at the senior level will be held personally accountable and face the consequences," Mr Ban said as the Security Council approved a surge of 5,500 new peacekeepers to bolster the 7,000-strong force already in South Sudan. Officially, 81,000 people have fled their homes and 500 have died since soldiers loyal to the former vice-president mutinied and began a firefight in an army barracks in Juba, the capital 10 days ago. Since then the fighting has spread across half of South Sudan's 10 federal states, including those where the country's oil fields lie. There are fears that rebel forces could seize control of those supplies, allowing them effectively to hold the government to ransom and giving them a significantly better hand should peace talks take place. Talks look to be a distant prospect, however. Riek Machar, the former vice-president who heads the insurrectionists, said he would negotiate only if his allies were freed from jail, where they have been held since the fighting flared. Salva Kiir, South Sudan's president, would not agree to those "pre-conditions" to talks, his government spokesman said Monday, leaving the political situation deadlocked and further violence expected. That death toll of 500 is widely believed to be a serious under-estimation. Toby Lanzer, the UN's humanitarian coordinator for South Sudan, said he had witnessed "some of the most horrible things that one can imagine" during a visit to Bor, a city north of Juba that fell to the rebels last week. Mr Lanzer added that there was "absolutely no doubt in my mind that we're into the thousands" of dead, the first clear indication of the scale of the conflict engulfing the world's youngest country, which became independent of its former civil war foe Sudan in July 2011. Navi Pillay, the UN special rapporteur on civil rights, said yesterday that a mass grave had been found in the rebel-held town of Bentiu and cited reports of at least two more in Juba. Around 15 bodies were found in one site in Bentiu, with another 20 bodies at a nearby river. In Juba, the UN mission was more cautious, confirming the 15 killed but saying it was still "investigations reports of such atrocities." The violence pits fighters from Mr Machar's Nuer people against Mr Kiir's Dinkas, the two largest tribes in the country. But the motivations for the violence appear more political than tribal. The rift between the two men worsened in July when Kiir fired Machar and several of his ministerial allies. Since then, Machar has said that the president is turning increasingly dictatorial and has promised to stand against him in elections due in 2015.