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* FARC will not agree to unilateral ceasefire
* If talks fail war could go on "another 50 years"
By Jeff Franks
Havana, Sept 13 (Reuters) - Colombia's FARC rebels believe the chances of achieving peace in upcoming talks with the government are high, but failure could bring many years more of war, a senior FARC leader said on Thursday.
Jesus Carvajalino, known by his war alias Andres Paris, said there are significant obstacles to reaching an accord, but after five decades of conflict Colombians so yearn for peace he thinks they can be overcome.
The FARC, or Revolutionary Armed Forces of Colombia, and the Colombian government will start talks in Oslo, then move to Havana. The rebels say they will begin Oct. 8, but the Colombian government says no official date has been set.
"If we compare the levels or expectations of support there is now for the talks, [the chances of peace] are 75 percent, 80 percent," Paris told Reuters in an interview in the Cuban capital, wearing a short-sleeve shirt and jeans, not a military outfit.
"But this probability can change if those forces against the process impose themselves and begin to fire away against the talks," said Paris, who is a member of the FARC delegation that will attend the talks.
If these talks are unsuccessful as previous attempts at peace have been, the war that has racked the Andean nation for so long could go on for decades because the two sides are basically at a stalemate, he warned.
"The idea that they have to kill all the guerrillas is what has perpetuated this conflict for 50 years. If they continue persisting in finishing off the conflict, we will extend it another 50 years," he said.
Some experts believe FARC is anxious to reach a peace accord because it has been weakened by a U.S.-backed Colombian military offensive that has thinned FARC's ranks, damaged their communications and pushed them into increasingly remote areas.
But Paris called the idea of FARC weakness "an invention," and insisted that the rebel group is as strong as ever.
Leaders of the FARC have said they will start the upcoming talks seeking a bilateral ceasefire, but Colombian President Juan Manuel Santos rejected the idea and said instead he plans to intensify military operations during the peace process.
Paris said FARC would not declare a unilateral ceasefire, saying it would be meaningless.
"They are going to continue offensives and our soldiers will have to respond equally to their attacks. So the ceasefire is bilateral or it's not possible," he said.
Colombia's conservative former President Alvaro Uribe and former ally of Santos has criticized the peace talks saying they could scare off investment that has contributed to the nation's economic boom.
"The president has allowed security to weaken and spent the last two years (negotiating) under the table, behind the back of the nation, while security has declined and the military is demotivated," Uribe told Reuters in an interview at his residence near the city of Medellin.
Paris said peace would allow Colombia to fully develop its economy, which he said had been crippled for years by the conflict, and he attacked Uribe for opposing the peace process.
"Uribe is emerging as the champion against the peace and for the way of violence, the extermination of the insurgency and every kind of social protest," he said
A decade ago, in the last attempt to end the war that has taken tens of thousands of lives, the government agreed to create a demilitarized zone the size of Switzerland which the rebels used to beef up their military and profit from a major drug-trafficking network.
The two sides have set a agenda for talks that includes the rights of victims, land ownership in rural areas and cocaine production and smuggling.
Paris said the FARC expects the biggest fight over its proposals for social change, including the setting aside of almost 25 million acres (10 million hectares) of land for peasant farmers.
"In the guts of our confrontation is the demand that there be changes in the economic and social areas. There is where we see the most resistance," he said. (Additional reporting by Helen Murphy; Editing by David Adams and Cynthia Osterman)