* U.N. rejects Gbagbo victory, backs Ouattara
* Ouattara's opposition camp warns of return to civil war
* Cocoa futures rise more than two percent
(Recasts)
By Tim Cocks and David Lewis
ABIDJAN, Dec 3 (Reuters) - The United Nations hailed Alassane Ouattara as the winner of Ivory Coast's election on Friday, dismissing amended results that gave victory to incumbent Laurent Gbagbo and sparked fears of renewed strife.
Ex-IMF official Ouattara claimed the win and was endorsed by northern rebels who fought Gbagbo in a 2002-2003 civil war, showing how Sunday's poll has accentuated the north-south divide in the world's top cocoa grower rather than heal it.
Highlighting the risk of fresh unrest, security forces used teargas to disperse protesting Ouattara supporters in the economic capital Abidjan. A Reuters reporter saw the Republican Guard deployed in the town centre around Gbagbo's palace.
In a statement, U.N. spokesman Martin Nesirky said U.N. Secretary-General Ban Ki-moon urged Gbagbo to help ensure a smooth transition of power.
"The Secretary-General congratulates Mr. Alassane Ouattara ... on his election and calls upon the President-elect to work towards lasting peace, stability and reconciliation in Cote d'Ivoire (Ivory Coast)," Nesirky said.
"The Secretary-General also calls upon President Laurent Gbagbo to do his part for the good of the country and to cooperate in a smooth political transition in the country," he added.
The continental African Union grouping issued a statement saying it was "deeply concerned" by developments in Ivory Coast.
Earlier, Ivory Coast's Constitutional Council -- led by a staunch Gbagbo ally -- upheld his complaints that the vote had been rigged by pro-Ouattara rebels in the north and declared him the re-elected president.
But the local United Nations mission in Ivory Coast refused to approve Gbagbo's re-election as president as required under existing peace accords, and instead backed provisional results giving Ouattara victory with 54.1 percent.
"I am sorry for the image of my country, but the end of the process is the validation of the UN special representative, and it is that validation which confirms I am the victor," Ouattara told reporters, urging calm and for the army to be neutral.
Neither Gbagbo nor his campaign team were immediately available for comment on developments.
In a statement issued from their stronghold in the northern town of Bouake, the rebel New Forces said they backed Ouattara.
Rebel forces in the north had in principle agreed to disarm as part of the peace process before the vote but they remain in control of the north and many have not given up their weapons.
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The U.N. endorsement of Ouattara raises numerous questions about how far the world body, which has some 10,000 peacekeepers and police in the country, is ready to go if Gbagbo insists on remaining in power despite huge international pressure.
The uncertainty propelled cocoa futures higher, with the March contract <LCCc2> ending 36 pounds higher at 1,959 pounds a tonne. However some analysts suggested the move was overdone and noted that even during the war, cocoa came out of the country.
TARGETED SANCTIONS?
Allies of Ouattara warned earlier of a possible return to war if the Constitutional Council, headed by Gbagbo party ally Paul Yao N'dre, overturned the provisional result -- a move he promptly announced minutes later on state television.
"By doing that they will cement the division of the country ... If Yao N'Dre does it he will be to blame for the next war in Ivory Coast," said Ouattara aide Jeannot Ahoussou.
Yao N'Dre cancelled votes from four regions in the north, giving Gbagbo 51 percent of the total vote.
"This is typical of Gbagbo ... Unfortunately, it puts the country back into a potential conflict zone," Tara O'Connor of London-based Africa Risk Consulting said of Gbagbo's perceived reluctance to leave office.
O'Connor suggested the United Nations could push for "targeted sanctions" on his leadership -- measures that can include travel bans or foreign assets freezes on individuals and which have been used on rogue leaders with varying success.
"The next thing will be to watch how many African leaders come out to congratulate Gbagbo on his win," said a diplomat in Abidjan. "Sanctions are definitely on the cards. They could move very quickly."
March cocoa futures <CCH1> rose ${esc.dollar}67 or 2.3 percent to finish at ${esc.dollar}2,935 per tonne, the highest settlement since Nov. 18.
Ivory Coast's ${esc.dollar}2.3 billion Eurobond <CI049648839=>, a bellwether of recovery hopes for what used to be one of the region's star performing economies, yielded 10.97 percent on Friday, up from pre-vote levels of below 10 percent. (Additional reporting by Ange Aboa in Bouake and Loucoumane Coulibaly in Abidjan; Louis Charbonneau at the United Nations; Nigel Hunt in London and Richard Valdmanis in Dakar; writing by Mark John; Editing by Richard Valdmanis and Jon Boyle)
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