GENEVA (Reuters) - South Sudan s hellish war will get worse if its neighbors let President Salva Kiir hold an election before he accepts peacekeepers, a ceasefire and political opposition, U.N. investigators said on Wednesday. South Sudan was plunged into civil war in 2013 when Kiir fell out with his deputy, Riek Machar. Since then 50,000-300,000 people have died, according to the African Union (AU), and millions have been displaced or pushed to the brink of famine.  It s just a story of absolutely unimaginable cruelty,  Yasmin Sooka, head of the U.N. Commission on Human Rights in South Sudan, told a news conference in Geneva.  Last year Machar fled, but during his retreat Kiir s forces sparked a new conflict in the Equatorias region, boosting opposition to Kiir s Dinka tribe, which almost one in four South Sudanese belong to. Kiir s mandate expires in April, and he has made clear he wants a new election, which would be catastrophic, Sooka and her fellow commissioner Godfrey Musila said.   You will not have a democratic Dinka president elected,  Musila said.  If the Dinka are to stay in power they will have to do that militarily. It has to be a police state.  Outside powers must urgently find a way to generate a legitimate government, he said. The first task was to fully deploy a 4,000-strong regional protection force that Kiir and East African bloc IGAD agreed on a year ago. Although limited to the capital, Juba, the force would create conditions for the opposition to return and would free up U.N. peacekeepers to go further afield, helping to bring about a ceasefire, which the opposition, with no military supply lines, would accept, Musila said.  Only 400 of the force have arrived so far. Sooka said the best tool to get action was the  hybrid court  that the AU and South Sudan agreed to set up, based on a mix of their laws, to prosecute suspects including top political and military figures. Sooka has an  almost prosecutorial mandate  from the U.N. Human Rights Council and plans to have a team of 16 on the ground this week, including forensic analysts and military advisers. On Monday, South Sudan Justice Minister Paulino Wanawilla Unango told the Council that the government was still incorporating last year s agreement with IGAD.  Sooka said the AU must ensure consequences if the government failed to take the necessary steps, as it did in Gambia in January, when it intervened to oust president Yahya Jammeh.   The West Africans camped outside of Gambia and told the president if you don t go, our armies will come in,  she said.  But will the East African community or IGAD do that?  