An operation by Venezuelan helicopters to collect three hostages due to be released in Colombia by the Farc rebel group has been delayed for a third day.
The negotiations have been punctuated by diplomatic spats
Venezuelan officials said the group had not provided the co-ordinates for the handover and that there was not enough time to complete the mission on Sunday.
The rebels have promised to release the hostages as a humanitarian gesture to Venezuelan President Hugo Chavez.
Mr Chavez said on Saturday that he hoped they would be released by Monday.
"They are still waiting for the details to complete the operation," he told Venezuelan state television.
The three hostages - two Colombian women and a young boy born to one of them in captivity - are among more than 40 high-profile detainees held by the Revolutionary Armed Forces of Colombia (Farc).
Several hundred hostages are being held by the group overall, some of whom are being held for political leverage but many also for ransom.
Two military helicopters carrying Red Cross insignia landed in the central Colombian town of Villavicencio on Friday.
On Saturday, an international commission of observers from seven countries, including the American director, Oliver Stone, arrived ready to accompany the aircraft when they pick up the hostages.
"Everything is ready, all we're waiting for are the co-ordinates," said Ramon Rodriguez Chacin, a former Venezuelan minister who is in charge of the operation.
"But it must be understood that the [Farc] patrol accompanying the hostages has to take precautions," he added.
Mr Rodriguez said that once the handover was complete, it was likely that Colombian military operations against the rebels would resume.
"The guerrillas should also expect that and prepare their retreat strategy and take all the security measures they need," he said. "That takes some time."
The hostage mission has also been restricted to daylight hours by the International Committee of the Red Cross (ICRC) because of its internal safety procedures.
"We don't have the time to start and finish the operation today," a senior Venezuelan official told the Reuters news agency on Sunday afternoon.
The hostages due to be freed are Clara Rojas, an aide to ex-presidential candidate Ingrid Betancourt, with whom she was kidnapped in 2002, and Ms Rojas's son, Emmanuel, said to have been fathered by one of her captors.
The other captive is former congresswoman Consuelo Gonzalez de Perdomo, who was kidnapped in 2001.
Fifteen members of the hostages' families, who have not seen their loved ones for more than five years, are waiting in the Venezuelan capital, Caracas.
President Chavez's efforts to negotiate the exchange of the remaining 43 hostages for some 500 guerrillas imprisoned in Colombian jails have been rejected by the Colombian government.
Several hundred hostages overall are being held by the Farc
He was involved in negotiations between the Farc and Colombia for months until he was told he had overstepped his mandate.
President Alvaro Uribe said the Venezuelan leader had been in direct contact with Colombia's army chief, despite being told explicitly not to do so.
In response, Mr Chavez said he would freeze Venezuela's bilateral ties with its neighbour and close trading partner.
The BBC's Jeremy McDermott in Medellin says sources in the Colombian government say it wants to regain the initiative with respect to the prisoner exchange and does not want Mr Chavez, who is perceived as being too friendly with the Farc, to hijack negotiations.