Helicopters sent by Venezuelan President Hugo Chavez to collect three hostages due to be freed by left-wing Farc rebels have landed in Colombia.
The two helicopters, carrying Red Cross insignia, landed in the central Colombian town of Villavicencio.
The rebels said last week they would hand over the hostages to Mr Chavez but the plan has been delayed.
Mr Chavez acted as a mediator between Farc and Colombia for months until Bogota said he overstepped his mandate.
He said the main rescue operation, codenamed "Emmanuel", had been delayed because Farc had not yet disclosed the precise location of the captives.
"The information is that there is bad weather, they are travelling and they have difficulty communicating in the jungle," Mr Chavez told Reuters news agency.
The hostages expected to be freed are Clara Rojas, captured during her 2002 vice presidential campaign, former lawmaker Consuelo Gonzalez de Perdomo, taken the year before, and Ms Rojas' son Emmanuel, said to have been fathered by one of her rebel captors about four years ago.
Among the observers present as the helicopters took off from western Venezuela were former Argentine President Nestor Kirchner and American film director Oliver Stone - a fan of Mr Chavez who was in Venezuela to make a documentary.
Mr Chavez said Mr Kirchner and other observers would follow the helicopters to Villavicencio as soon as the Venezuelans receive word from the guerrillas about where to pick up the hostages.
He said the rebels had demanded that the Venezuelan pilots not be told where they will fly until they were airborne.
The three hostages are among some 45 high-profile captives - including French-Colombian politician and former Colombian presidential candidate Ingrid Betancourt - who the guerrillas want to swap for some 500 rebels in Colombian jails.
There are several hundred hostages overall being held by the guerrillas.
Some are held for political leverage but many also for ransom by Farc, who have been fighting the Colombian government for more than four decades.
Colombian President Alvaro Uribe said he was thinking of the all those kidnapped or killed by the Farc.
"We keep waiting and hoping for all the hostages to be released... thinking about 750 Colombians kidnapped by the Farc in the last 10 years and haven't returned to their homes," Mr Uribe said at a press conference.
The Colombian government has said Venezuela has until 2359 GMT on Sunday to carry out the mission, although Mr Chavez has said he is not aware of any time limit.
Taking part in the handover mission will be representatives of the International Committee of the Red Cross and envoys from France, Switzerland, Argentina, Brazil, Bolivia, Cuba, Ecuador as well as Colombia.
Last week the Farc rebels announced they would release three hostages to Mr Chavez or "whomever he opts to designate".
Correspondents say the Farc rebels are hoping that through Mr Chavez's involvement they can force the Colombian government to make concessions.
In early December, Mr Uribe offered to designate a limited safe area to enable talks to take place aimed at exchanging rebel-held hostages for jailed rebels.
But the Farc want a larger zone in south-western Colombia to be demilitarised, a demand Mr Uribe has rejected. ( BBC )