(Adds Colombia statement, Chavez quotes, details)

By Francois Murphy

PARIS, Nov 20 (Reuters) - Colombian Marxist guerrillas have pledged to prove this year that high-profile hostages are alive, Venezuelan President Hugo Chavez said on Tuesday, and Bogota stated it could accept talks between Chavez and a rebel leader.

Chavez is mediating between Colombia's government and the Revolutionary Armed Forces of Colombia (FARC), a guerrilla group that has waged a four-decade war against the state.

He had hoped to bring his French counterpart Nicolas Sarkozy proof that French-Colombian politician Ingrid Betancourt, who was kidnapped by the FARC in 2002, is alive, but instead he said he had a pledge that such proof would be forthcoming this year.

"Ingrid is alive, I am absolutely certain of it," Chavez told reporters after meeting Sarkozy.

He said FARC commander Manuel Marulanda pledged to prove it in a letter sent to Colombian Senator Piedad Cordoba.

"We have received a letter from Mr Marulanda through Mrs Cordoba in which the head of the FARC pledges to provide proof of life before the end of the year, not only concerning Mrs Betancourt but the other hostages as well," Chavez said.

Chavez and Sarkozy met relatives of Betancourt, who was kidnapped along with her campaign aide Clara Rojas. Neither Betancourt nor Rojas have been heard from since 2003.

The FARC wants rebels held in government prisons to be freed in exchange for its most high-profile captives, including Betancourt and three U.S. contract workers held for years in jungle camps.

COLOMBIA SHIFTS

Colombia said on Tuesday it would authorize Chavez to meet Marulanda in a zone under international observation inside Colombia should the guerrillas first release a group of hostages in a sign of good faith.

"If there is a commitment to releasing all the hostages, then the meeting between President Chavez and Manuel Marulanda could take place," Colombia's Peace Commissioner Luis Carlos Restrepo told reporters in Bogota.

"What we want before the end of this year is to find a formula to free the hostages," he said.

The proposal was a shift for Colombian President Alvaro Uribe who had dismissed such a meeting in Colombia. Talks to free hostages are deadlocked over FARC demands for a demilitarized zone and for two rebels held in U.S. jails to be included in any hostage exchange.

However, Chavez indicated that Restrepo's statement was more restrictive than what had been agreed with Uribe at a meeting in Santiago earlier this month, at which he said the Colombian president told him freeing a first group was enough for talks.

"There was never a question of having a condition that they all be freed. That is absurd. What is the point of me going ... if they will all be freed beforehand?" Chavez said in Paris.

On Monday, Bogota also increased the pressure on Chavez by saying it was giving him until the end of the year to show progress in his mediation with the FARC.

"Impatience is not good counsel," Chavez responded, speaking through an interpreter who translated his comments into French.

"We have achieved things in three months that had never been achieved in five years previously," he said.

(Additional reporting by Patrick Markey in Bogota, Elizabeth Pineau in Paris)

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