Paul Reyes, a spokesman for the Revolutionary Armed Forces of Colombia (Farc), said on Thursday the rebel group had "the political will" to negotiate an exchange.
The organisation had previously rejected any talks with the conservative Colombian president, Alvaro Uribe, who has continued a US-backed military campaign against the rebels.
However, the rebel spokesman said the group would not hold any negotiations in territory occupied by government troops.
It is a key Farc demand that Colombian troops withdraw from two southern provinces.
"It is Alvaro Uribe who will decide whether to continue the war or seek to sit down with the FARC," Reyes said in an interview with the Telesur channel.
"The Farc won't accept talks under the table, in private, outside the country or any place in Colombia with this government until these areas are demilitarised."
Francisco Santos, the Colombian vice president, promised to study Reyes's remarks in detail but cautioned against reading too much into his words.
"A complex process like this requires prudence - it can't be handled with declarations before microphones," Santos said.
A government spokesman said Uribe's position, that he will neither cease anti-guerrilla operations nor demilitarise a large part of the country in order to hold talks, had not changed.
The government is asking the 17,000-strong Farc to free 62 hostages, including three American defence contractors and French-Colombian politician Ingrid Betancourt, in exchange for rebels held in government jails.
The National Liberation Army, or ELN, Colombia's second- biggest guerrilla group, is holding preliminary peace talks with Uribe's government in Cuba.
The FARC and ELN say they are fighting for socialism in a country with deep divisions between rich and poor, but mainstream politicians say the groups have scant popular support.