* Powers exasperated by broken ceasefires
* South Sudan spokesman says deal was "sell-out"
* Kiir unhappy with plans for capital, talks with rival (Adds comments by government on alleged clashes)
By Elias Biryabarema
KAMPALA, Aug 18 (Reuters) - 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 from a regional power 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 reached.
Kiir asked for another 15 days of discussions, shrugging off pressure from regional mediators to meet a Monday deadline for a deal. His spokesman told journalists in Juba on Tuesday the pact on the table had been a "sell-out", without going into details.
A procession of African leaders flew in to join the negotiations in Addis Ababa and press for an agreement, including Ugandan 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."
OIL, CHAOS
South Sudan, an oil producer which gained independence from Sudan in 2011, descended into chaos in December 2013 when a political dispute between Kiir and his deputy Riek Machar spiralled into armed conflict that reopened ethnic fault lines.
Machar, who signed the deal, accused government troops of launching attacks on opposition forces' positions just hours after Kiir snubbed the peace deal. "The regime in Juba rejected to sign the peace agreement because it has chosen war over peace," he said in a statement on Tuesday.
The government rejected those assertions, instead accusing the rebel forces of starting the clashes.
"These are lies. It is the rebels that have attacked Pageri town this morning at 10 a.m. The rebels set fire to civilian houses and left many casualties," Philip Aguer, spokesman for the military, said.
He said an assessment of the damage and casualties caused by the fighting in Pageri was being carried out by the military.
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.
A member of the mediation team from regional bloc IGAD told Reuters that Kiir, who only initialled the deal, rather than signing it as Machar did, had reservations about a provision in the plan to demilitarise the capital, Juba.
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, the official said.
"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. (Additional reporting by Denis Dumo in Juba and Aaron Maasho in Addis Ababa; Writing by Edith Honan and Duncan Miriri; Editing by Mark Heinrich)
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