The government has refused the FARC's demand for a two-way truce since the sides began peace talks in Cuba in late 2012, casting doubt on the viability of the rebel ceasefire if the government does not reciprocate.
"We have resolved to declare a unilateral ceasefire and end to hostilities for an indefinite time, which should transform into an armistice," the FARC, or Revolutionary Armed Forces of Colombia, said.
The group 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, as reports Report.
"This unilateral ceasefire, which we hope will be prolonged in time, will only end if it is determined that our guerrilla structures have been the object of an attack by the military," it said.
The group made an unusual demand for certification of its ceasefire through either the United Nations, the Red Cross, regional intergovernmental organizations or the Catholic Church.
The request may be designed to raise pressure on the government of President Juan Manuel Santos by putting the success or failure of the FARC's gesture down to the response of his administration, while the international community watches.
There was no immediate response from the government to the FARC's statement.
The FARC grew out of a 1960s peasant movement demanding land reform, and it has been fighting successive governments ever since in a conflict that has killed more than 220,000 and created one of the world's highest internally displaced populations.
This 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 demobilization and victim reparations.