Alvaro Uribe, Colombia's president, said such a zone would allow Farc to regroup in a similar way to what had happened when troops were pulled back by his predecessor.
On Tuesday, the government freed Rodrigo Granda, a high-ranking Farc leader whom Uribe wants to broker the exchange of rebels for hostages.
But Granda's attorney Miguel Angel Gonzalez said he lacked Farc authority to negotiate any prisoner exchange.
Some of the Farc fighters who are set to be freed from 50 prisons nationwide on Wednesday have been held for more than five years.
Granda's release was specifically requested by Nicolas Sarkozy, the French president, to secure the release of a prominent French-Colombian politician, Ingrid Betancourt.
France, Spain and Switzerland are all involved in efforts to broker a deal between the government and the Farc, which began as a peasant army in the 1960s and is now engaged in Colombia's cocaine trade.
The US on Tuesday urged Farc to release all its hostages, including three Americans, and reminded Uribe to "safely obtain" their freedom.
Eric Watnik, a state department spokesman, said: "We hold the Farc and other armed groups responsible for the health and welfare of the hostage US citizens."
Helped by billions of dollars in US aid, Uribe has sent troops to push back Farc into the jungle, take back areas once under their control and disarm thousands of illegal paramilitaries which had helped fight the rebels.
Details of life in secret fighter hideouts surfaced last month when a police officer who escaped after nine years in captivity said he was held with Betancourt and the Americans until he fled at the end of April.