CARACAS, Nov 8 (Reuters) - Venezuelan President Hugo Chavez and a Colombian Marxist guerrilla commander were upbeat on Thursday after initial talks to negotiate the release of rebel hostages, including three American men and a French woman.

"Today I'm more optimistic than ever over this humanitarian exchange issue," said Chavez, dressed in military uniform with FARC commander Ivan Marquez at his side at the presidential palace.

The guerrilla expressed confidence Chavez's mediation could break an impasse, saying if he and FARC leader Manuel Marulanda were to meet as they hope to they could "move the unmovable."

The leftist pedigree of an ex-soldier who led a failed coup in 1992 gives Chavez credibility with Latin America's oldest insurgent group, the Revolutionary Armed Forces of Colombia.

But it is unclear if the talks he began this week can overcome persistent obstacles, such as the Colombian government's refusal to grant the rebels a demilitarized zone.

The FARC wants rebel prisoners freed in exchange for their most high-profile captives, including French-Colombian politician Ingrid Betancourt and three U.S. contract workers held for years in jungle camps.

Chavez said the guerrillas have pledged to provide proof the hostages are alive, something diplomats say is a key first step to making progress in talks.

(Reporting by Brian Ellsworth; editing by Saul Hudson and Todd Eastham)

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