A Venezuelan-led mission to free three hostages held by Colombian left-wing Farc rebels has been suspended.
The negotiations have been punctuated by diplomatic spats
The rebels said the planned release was not possible because of government military operations, according to Venezuelan President Hugo Chavez.
But the Colombian president Alvaro Uribe said no new operations were under way and that the rebels may not be in possession of one of the hostages.
The Farc had promised Mr Chavez that they would release two women and a boy.
Venezuelan military helicopters that were to have collected the hostages are now on their way back home.
Two of those slated to be freed were 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 third was former congresswoman Consuelo Gonzalez de Perdomo, who was kidnapped in 2001.
Child 'in Bogota'
On Monday, the Venezuelan leader read out what he said was a letter from the Farc on television.
Several hundred hostages overall are being held by the Farc
Military operations "impede us for now from turning over" the three hostages, the Farc were quoted as saying.
But President Uribe accused the rebels of lying, and said the boy may actually be in a children's home in the Colombian capital, Bogota.
"The Farc can't keep the promise to free the hostages because they no longer have the child, Emmanuel, in their power," he said.
President Uribe has asked relatives of Ms Rojas for DNA samples to prove that a three-year-old boy in the Colombian capital is really the missing Emmanuel.
The planned handover operation had to be postponed on Sunday when Venezuelan officials said the Farc had not provided the co-ordinates for the handover and that there was not enough time to complete the mission.
Fifteen members of the hostages' families, who have not seen their loved ones for more than five years, have been waiting in the Venezuelan capital, Caracas.
Mr Chavez has been trying to negotiate an exchange of other hostages for guerrillas imprisoned in Colombian jails but he has been accused by the Colombian authorities of overstepping his mandate as a mediator.
In response, he has threatened to freeze ties with the neighbouring state, which is a close trading partner.
It is believed that the Colombian government wants to regain the initiative with respect to the prisoner exchange and does not want Mr Chavez, perceived as being too friendly with the Farc, to hijack negotiations.