Colombia's FARC rebels have declared an indefinite, unilateral ceasefire to start on December 20, saying they will only use weapons if they came under attack by the army.

"We have resolved to declare a unilateral ceasefire and end hostilities for an indefinite period of time, which should be transformed into an armistice," said the leftist rebels' peace negotiators in Cuba, where they are in talks with the Colombian government to end the 50-year war.

"This unilateral ceasefire, which we hope will last a long time, will end only if our guerrilla units have been the object of attacks by the security forces".

We want to overcome these useless bloody episodes. FARC statement

The FARC have repeatedly called for a bilateral ceasefire as part of the ongoing peace process.

Last month, FARC freed an army general and two other hostages they had captured, paving the way for peace talks to resume.

President Juan Manuel Santos had halted peace negotiations in the Cuban capital Havana after the rebels seized five hostages, including General Ruben Dario Alzate. The move disrupted efforts to stop Latin America’s longest-running war that has killed more than 200,000 people since 1964.

The suspension of talks was the most serious setback to peace efforts after months of discussions and partial accords.

FARC has declared temporary ceasefires during the past two Christmases and it halted combat operations during this year's elections, but has not called an indefinite ceasefire since the current talks began.

The FARC grew out of a 1960s peasant movement demanding land reform, and it has been fighting successive governments ever since.

The ongoing peace process has gone further than previous attempts with partial deals already on land reform, the FARC's political participation and ending the illegal drugs trade. The sides are now entering a delicate phase as they debate the thorny issues of rebel demobilisation and victim reparations.

"The coming months are fundamental. ... It's nothing more nor less than a now or never scenario," the statement said. "We want to overcome these useless bloody episodes."