A senior guerrilla recently freed by the government in hopes of advancing a hostage exchange may play a role in the possible deal.
But Thursday's statement by the Revolutionary Armed Forces of Colombia pointedly avoided suggestions by the government and hostages' relatives that Rodrigo Granda might negotiate a swap of rebel hostages for government prisoners.
It said only that Granda could confirm the existence of a demilitarized zone that leaders of the group, known as the FARC, insist be part of any hostage deal to ensure their safety. The Colombian government has repeatedly rejected the idea.
"If the national government commits to totally freeing Rodrigo Granda, and guarantees his freedom to move and directly contact our secretariat ... we will consider him as the FARC's verifier of the demilitarized zone of Florida and Pradera," the FARC said Thursday, referring to southwest municipalities totaling about 800 square kilometers (310 square miles).
It was the FARC's first formal statement since the release this month of Granda, who was the most senior incarcerated rebel.
Before going to Cuba this week, Granda told The Associated Press that he had not spoken with rebel commanders because he was afraid the government was spying on him.
President Alvaro Uribe repeated his long-standing opposition to the zone on Thursday, saying it would "not be beneficial for the country."
Granda, known as the "foreign minister of the FARC," was released in what Uribe called a "gesture of good faith." But the FARC dismissed the government's suggestion he serve as a negotiator, and instead named three commanders camped out in the jungle.
The FARC is offering to exchange around 60 hostages, including former presidential candidate Ingrid Betancourt and three kidnapped U.S. defense contractors, for all their imprisoned comrades. The government has agreed in principle to the swap, but rejects the zone and insists the freed guerrillas leave the FARC.