Luis Carlos Restrepo, Colombia's high commissioner for peace, said: "The most important thing is the offer from these congress members to go to the negotiation zone as guarantors.

 

"We believe this could help give confidence to Farc, which has always been afraid to enter talks."

 

Farc, concerned for the safety of its negotiators, has asked that Colombia withdraw government troops from a rural area, nearly the size of New York City, to negotiate the exchange.

 

"We believe this could help give confidence to Farc, which has always been afraid to enter talks"

Luis Carlos Restrepo, Colombian peace negotiator

Alvaro Uribe, the Colombian president, says he is considering the idea but may instead try to rescue the kidnap victims, an option rejected by families of the hostages as too dangerous.

 

Thomas Howes, Keith Stansell and Marc Gonsalves, all US defence contractors, were captured after their aircraft crashed in the jungle during a drug-eradication mission in 2003.

 

Farc is also holding Ingrid Betancourt, a French-Colombian taken during her 2002 presidential campaign.

 

James McGovern, a Massachusetts Democrat, said: "I believe the way to reunite these families is through an exchange.

 

"If we can do that it could be a step toward other talks aimed at bringing an end to the conflict."

 

An estimated 3,170 hostages are held by the Farc, other rebel groups and criminals in Colombia, according to government figures.

 

Farc grew out of a campaign to force land reforms and other measures aimed at reducing the gap between rich and poor in the 1960s.