Uribe suggested the rebels could be backing out of the deal, brokered by Chavez, because they do not after all have the boy hostage.
"The Farc terrorist group doesn't have any excuse. They've fooled Colombia and now they want to fool the international community," he said.
Uribe also made the suggestion that Farc fighters "don't dare to keep their promises because they don't have the boy, Emmanuel".
Uribe said his government had given Venezuela and the International Red Cross co-ordinating the mission every guarantee that its military would not obstruct the handover.
He said that included a promise to create a ceasefire corridor to allow the rebels to escort their hostages through the jungles to the pickup point.
Following the news, Nestor Kirchner, the former Argentine president, and observers from France, Switzerland and four Latin American governments left the Colombian town of Villavicencio on Monday.
|The deal to free the hostages had |
been brokered by Chavez [Reuters]
They said they would "continue their mission" once all conditions for the hostages' release were met.
Oliver Stone, the US filmmaker who had been invited by Chavez to document the handover, criticised the Colombian authorities before boarding a flight back to Caracas.
"Shame on Colombia, shame on Uribe," Stone said.
He said the Farc leaders "have no motive not to release these hostages".
The Farc in its letter to the Venezuelan leader said "military operational attempts" had prevented it from turning over the three hostages.
Venezuelan helicopters have been waiting in Columbia since Friday for the group to provide the location where the hostages could be picked up.
Colombian officials have denied interfering in the airlift mission, and Colombia's top peace envoy blamed the delays on the Farc.
Chavez said the Farc insisted in the letter that the current plans for a handover would put the lives of the hostages and members of its group at risk.
Colombia's government accused the Farc of lying and dragging their feet.
Uribe said on Monday that a three-year-old child named Juan David Gomez, matching the description of Emmanuel provided by escaped hostages, may have been living for the past two and a half years with at a foster home in Bogota.
The child was suffering from malnutrition, malaria and jungle-born leishmaniasis at the time he was turned over.
Gomez was turned over in the eastern city of San Jose del Guaviare, a Farc stronghold, in 2005 by a man who said he was the boy's great uncle and who now claims to be his father.
The boy's mother was reported as disappeared, according to the child welfare agency case file read to journalists by peace commissioner Luis Carlos Restrepo.
Uribe said only DNA tests were required to prove or disprove "this hypothesis" - which he said could be done as soon as the boy's grandmother returns from Caracas, where she was awaiting the handover of her daughter and grandson.
The international Red Cross confirmed Uribe's claim that his government informed it of the child's existence two days ago.