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By Hugh Bronstein

BOGOTA, Dec 26 (Reuters) - Colombia agreed on Wednesday to allow Venezuelan aircraft to land in the country and pick up three hostages held by Marxist rebels, a breakthrough that could win the captives' quick release.

Venezuela's leftist President Hugo Chavez will send planes and helicopters to the Colombian town of Villavicencio at the foot of the Andes and then dispatch helicopters to a rebel camp to collect the hostages, including a former vice presidential candidate and her son, who was born in captivity.

Chavez, who has been negotiating with rebel leaders and is the only one authorized by them to receive the hostages, unveiled his rescue plan earlier on Wednesdsay and Colombia's conservative government quickly backed it.

"We authorize this humanitarian mission on its own terms," Foreign Minister Fernando Araujo said.

Chavez said the aircraft will bear the insignia of the Red Cross, and that the operation to pick up the hostages and fly them to Venezuela could begin within hours of receiving Colombia's authorization.

But Yves Heller, spokesman for the International Committee of the Red Cross in Colombia, said the hand-over could still be delayed by logistical and security details.

The three hostages are Clara Rojas, captured during her 2002 vice presidential campaign, and her young son Emmanuel, fathered in captivity by one of the rebels, as well as former lawmaker Consuelo Gonzalez, who was kidnapped in 2001.

Chavez said he wanted international observers to witness the hostages' release.

"We have different options for a clandestine hand-over, but we don't want that, it would be very risky," Chavez told reporters in Caracas. "All sorts of unexpected things could happen. These are mountains and jungles."

Colombian President Alvaro Uribe last month told Chavez to stay out of hostage negotiations with the guerrilla leaders but the anti-American leader continued to talk with them, and they said they would turn the three hostages over only to him or someone he designates.

Rojas's brother Ivan Rojas backed Chavez's plan. "We are very optimistic that this will turn out well," he told reporters in Bogota.

HIGH-PROFILE HOSTAGES

The release of the three hostages could help set the stage for an exchange of other kidnap victims, including three U.S. contractors and French-Colombian politician Ingrid Betancourt, for Colombian guerrillas locked in government jails.

Betancourt was a presidential candidate in 2002 when she and Rojas, her running mate, were captured by the Revolutionary Armed Forces of Colombia, or FARC, which has been fighting the state since the 1960s.

The three Americans -- Thomas Howes, Keith Stansell and Marc Gonsalves -- were captured during a mission to spot illicit coca crops in 2003.

French President Nicolas Sarkozy has pushed hard for Betancourt's release and he thanked Chavez and Uribe for the new breakthrough, which could boost the chances of her being freed in the future.

Argentina said former President Nestor Kirchner will travel to Colombia on Thursday as part of an international mission to ensure the success of the hostage turnover.

The FARC last made a high profile hostage release in 2001, when it set free 242 police and soldiers from jungle camps in a gesture aimed at reviving peace talks.

But those negotiations failed and the FARC continued to seize hostages, using them for ransom payments or to push for the release of imprisoned fighters.

Videos released in early December revealed the harsh conditions in which the hostages live. One showed Betancourt looking gaunt and depressed in a secret jungle camp.

Uribe, popular for his U.S.-backed crackdown on the FARC, has offered to designate a limited safe area to swap these and dozens of other high-profile captives for jailed rebels.

But the FARC insists he pull troops from a larger zone of its choosing to facilitate an exchange. The rebels want to enter that zone armed, which Uribe says he will not allow.

The FARC has been pushed onto the defensive by Uribe but it still controls wide rural areas and holds about 750 hostages for ransom and political leverage. (Additional reporting by Frank Jack Daniel in Caracas and Lucas Bergman in Buenos Aires; Editing by Kieran Murray)

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