Venezuelan President Hugo Chavez, who is acting as a mediator in Colombia's hostage crisis, said Tuesday that leftist rebels insist that the Colombian government create a demilitarized zone for a prisoner swap.

Chavez said the two top leaders of the Revolutionary Armed Forces of Colombia (FARC) guerrilla group, which wants to swap 45 high-profile hostages for 500 imprisoned rebels, made their demand in two letters sent to him.

The rebel hostages include dual nationality French-Colombian politician Ingrid Betancourt, three US citizens, and a number of Colombian senators, governors, congressmen and mayors.

"Mr. President, there is willingness within FARC to facilitate the exchange of prisoners held by FARC," Chavez read. One letter was written by FARC chief Manuel Marulanda and the other by his top deputy, Raul Reyes.

"For that to happen, we consider the removal of troops from the (southwestern) towns of Pradera and Florida essential to allow leaders of FARC and the government to agree on the terms and procedures regarding the captives," the letter said, according to Chavez.

But the government of Colombian President Alvaro Uribe reiterated Tuesday its refusal to create a demilitarized zone.

"This is not negotiable," Jose Obdulio Gaviria, a top aide of Uribe, told AFP. "We will keep giving the same answer."

Chavez said Reyes also wrote that FARC were "preparing the terrain" to hold a meeting between Chavez and Marulanda in Colombia.

Chavez had asked Uribe on Saturday to let him meet with Marulanda in Colombia's jungle, but Bogota denied the request.