Helicopters sent by Venezuelan President Hugo Chavez to collect three hostages due to be freed by left-wing Farc rebels have landed in Colombia.
Chavez (R) has been closely involved in negotiations with Farc
The two helicopters, carrying Red Cross insignia, landed in the central Colombian town of Villavicencio.
Mr Chavez said the flight was the first phase of a mission that would be completed over the weekend.
He was involved in negotiations between Farc and Colombia for months until Bogota said he 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 hostages expected to be freed are Clara Rojas, a former vice-presidential candidate who was kidnapped with Ingrid Betancourt in 2002, former congresswoman Consuelo Gonzalez, seized in 2001, and Ms Rojas' son Emmanuel, said to have been fathered by one of her captors four years ago.
Among the observers present as the helicopters took off from western Venezuela on operation "Emmanuel" were former Argentine President Nestor Kirchner and the American film director, Oliver Stone - a fan of Mr Chavez who is 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 received word from the guerrillas about where to pick up the hostages.
However, he warned that the pick-up could not take place immediately because the Farc had not yet disclosed the precise location of the captives and the Red Cross preferred not to operate during the hours of darkness for security reasons.
"I hope there will be good weather [on Saturday], that we can fill in some small details that are missing and I hope that tomorrow we can complete the operation," he said.
"If there were any problem finding the spot, for some military or weather reason, we would be ready to conduct ground operations, but for that we would need permission," he added.
The three hostages being released in what the Farc have described as a humanitarian gesture are among some 45 high-profile captives, who the group want to swap for around 500 guerrillas imprisoned by the Colombian authorities.
Several hundred hostages overall are being held by the group.
Some are held for political leverage but many also for ransom by the Farc, which has been fighting the Colombian government for more than four decades.
After hearing of the helicopters' departure, 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.
Several hundred hostages overall are being held by the Farc
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.