A meeting at which Venezuela President Hugo Chavez and Colombian rebels were to discuss an exchange of prisoners has been postponed indefinitely.
Relatives of hostages have urged Chavez to broker a deal
Mr Chavez hoped to broker an exchange of about 500 Farc rebels detained in Colombia for 45 important captives held by Farc at the meeting on 8 October.
But a mediator now says important issues have not yet been agreed.
The Farc are banned from flying to Caracas, while Colombia's Alvaro Uribe has ruled out a meeting in Colombia.
Earlier, President Uribe rejected proposals that rebels imprisoned in the United States should be included in any exchange of prisoners with his government.
Colombian opposition Senator Piedad Cordoba, who has been acting as a mediator in the process, confirmed that elements were lacking to "complete the circle" and reach a deal.
Senator Cordoba (right) has been working to secure a deal
The proposed deal would see the 500 rebel prisoners exchanged for politicians, members of the armed forces, and three US citizens.
The process is being closely watched by the French government, whose concern centres on the fate of French-Colombian politician Ingrid Betancourt, who was kidnapped five years ago.
Members of the left-wing Farc group, which has been waging a war against the Colombian government since the 1960s, have complained that the limitations set by President Uribe are hampering negotiations.
But President Uribe, who initially agreed to Mr Chavez's mediation, said an exchange that included the return to Colombia of two senior members of the Farc, known as Simon Trinidad and Sonia, imprisoned in the US could set a negative precedent.
Earlier, President Chavez told Venezuelan TV that other governments were supporting the process and he was "confident that everything will turn out fine".