• KAMPALA/JUBA (Reuters) - Regional power Uganda told South Sudan's warring factions on Tuesday to put their egos aside and make peace, a day after President Salva Kiir refused to sign a deal to end a 20-month-old conflict.

    The blunt words underlined growing exasperation among African and global leaders over a string of broken ceasefires and accords in the world's newest nation. Washington has threatened sanctions if no deal is sealed.

    Kiir asked for another 15 days of discussions, shrugging off pressure from regional mediators to meet a Monday deadline for an agreement. His spokesman told journalists in Juba on Tuesday the deal on the table had been a "sell-out", without going into details.

    A line-up of African leaders flew in to join the negotiations in Addis Ababa and press for an agreement, including Uganda's President Yoweri Museveni who at one stage stormed out of the venue.

    "The Ugandan government knows how strenuous it is to achieve peace between belligerents, especially when the belligerents have big egos and when those belligerents put their personal egos above national interests," Ugandan government spokesman Shaban Bantariza said in Kampala.

    "We can only continue to mediate, to encourage every side to realise that their country is superior to every one of them individually," he added.

    OIL, CHAOS

    South Sudan, an oil producer which gained independence from Sudan in 2011, descended into chaos in December 2013 when a political row between Kiir and his deputy Riek Machar spiralled into armed conflict that reopened ethnic faultlines.

    Rights groups have accused both sides of abuses in clashes and raids often pitting Machar's Nuer group against Kiir's Dinkas.

    Fighting has killed more than 10,000 people and forced more than 2 million to flee their homes in one of Africa's poorest countries.

    Machar signed the deal on Monday but Kiir only initialled it.

    A member of the mediation team from regional bloc IGAD told Reuters that Kiir had had reservations over the proposal's plan to demilitarise the capital, Juba.

    The official added Kiir had sought to scrap a provision that called for consultations with Machar on "powers, functions and responsibilities" he would exercise in any future administration.

    "We strongly believe that (the deal) cannot serve the people of South Sudan. It is a sell-out and we will not accept that," Kiir spokesman and South Sudan information minister Michael Makuei Lueth said on Tuesday.

    Western diplomats urged Kiir to accept the deal swiftly.

    "After 20 months of fighting, and with 2 million displaced from their homes, further delay beyond the 17 August deadline is completely unacceptable," Britain's minister for Middle East, North Africa and Central Asia, Tobias Ellwood, said in a statement.