Colombian rebels say will free two of 24 hostages

* FARC guerrillas call for comprehensive prisoner swap

* Rebels say an exchange could set stage for peace talks

BOGOTA, Sept 27 (Reuters) - Colombian rebels said on Sunday they will free two hostages in a unilateral gesture that could set the stage for a comprehensive exchange of prisoners held by both sides in the country's 45-year-old guerrilla war.

Soldiers Pablo Moncayo and Josue Calvo are locked up in secret jungle camps by Colombia's biggest guerrilla army, known as the FARC.

They and 22 other kidnapped members of the state security forces are being used as leverage by the Marxist rebels, who want to negotiate the freedom of hundreds of their fighters held in government jails.

"With this gesture of a unilateral release we reaffirm our willingness to advance in an exchange of all prisoners of war, whether they be held by the guerrillas or the state," said a statement issued by the outlawed FARC.

The swap could be a step toward talks aimed at ending the war, the group said. But President Alvaro Uribe's conservative government says the cocaine-financed guerrillas must lay down their arms and give up crime before peace talks can start.

Moncayo, snatched by the FARC in 1997, has become a symbol of the suffering of kidnap victims since his father Gustavo Moncayo began a campaign for his freedom, wrapping himself in chains and walking throughout Colombia.

Calvo was kidnapped earlier this year.

Opposition Senator Piedad Cordoba has been designated by Uribe to help mediate the release of the hostages, who at times appear stuck in a political tug of war between the leftist senator and the president.

Uribe, whose father was killed in a 1983 FARC kidnapping attempt, is popular for his U.S.-backed crackdown on the rebels. He may stand for a third term in 2010 if his supporters succeed in amending the constitution to allow him to run.

The decision to release the two soldiers was first announced by the FARC in April, but the plan got bogged down in disagreements with Uribe over the conditions for a hand-over.

The president agreed this month to FARC demands that it be allowed to free the 24 hostages one at a time rather than all at once, a reversal in policy that could speed releases.

Editing by Paul Simao