PARIS, Aug 30 (Reuters) - French President Nicolas Sarkozy has called left-wing Venezuelan leader Hugo Chavez to back his offer to try to help secure the release of hostages held by rebels in Colombia, including a French-Colombian politician.

Chavez has offered to act as an intermediary between Colombian guerrillas still fighting a 40-year conflict and the administration of conservative President Alvaro Uribe who has led a U.S.-backed military crackdown on the rebels.

The FARC, or Revolutionary Armed Forces of Colombia, has held hundreds of police, soldiers and politicians for years, including French-Colombian politician Ingrid Betancourt, seized in 2002, and three Americans kidnapped while on a counter-narcotics mission the year after.

Sarkozy has made securing Betancourt's release a priority.

"The president took the initiative yesterday to call his Venezuelan counterpart, President Hugo Chavez," Sarkozy's spokesman David Martinon told a news conference.

"The president assured President Chavez of his full support and of his interest in the initiative proposed by President Chavez and of his will to accompany his efforts, in agreement with Colombian President Alvaro Uribe," he added.

Uribe has fostered an image as a hard-nosed leader who has openly confronted the guerrilla groups and negotiated the surrender of right-wing paramilitary death squads in a campaign to reduce Colombia's violence.

The FARC want Uribe to demilitarize an area in southern Colombia the size of New York City to help talks over swapping key hostages for jailed rebels. He has released some guerrillas in a good-will gesture, but refuses to withdraw troops under the rebels' conditions.

U.S. officials have charged that Chavez, a self-styled socialist revolutionary, has openly aided FARC rebels, but they have not presented evidence to support the charges.

At Sarkozy's request, Uribe released a top FARC commander in June in an attempt to broker a deal on the hostages. Families are pressing Uribe to seek a humanitarian agreement and stop rescues by force.

Martinon said Sarkozy and Chavez had agreed to remain in touch, adding that the French president would call his Colombian counterpart on Thursday afternoon.

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