PARIS —  A French airplane landed in Colombia before dawn Thursday as part of a mission to help rebel hostage Ingrid Betancourt, who is believed to be gravely ill after more than six years in captivity, officials said.

Betancourt, a former Colombian presidential candidate who also has French citizenship, may be within hours of death if she does not get a blood transfusion, according to her son.

An official in French President Nicolas Sarkozy's office, speaking on condition of anonymity because of the sensitivity of the case, said an aircraft carrying French envoys had arrived in Colombia, without elaborating.

The plane arrived to Bogota at 1 a.m. local time (2 a.m. EDT), according to an official in Colombia's civil aviation authority who spoke on condition of anonymity because he was not authorized to speak on the subject.

French officials have been tightlipped about the mission. France-Info radio said the aircraft carrying at least three envoys left Wednesday from a military airport outside Paris. Officials said a doctor was one of the envoys.

French Foreign Ministry spokeswoman Pascale Andreani declined to provide details about the mission at a regular news briefing Thursday, saying: "Discretion is required in this type of case."

Betancourt is among hundreds of hostages held by the Revolutionary Armed Forces of Colombia, or FARC, but her release has become a cause celebre in France.

"The first objective of this mission is to succeed in getting close to Ingrid and care for her," Betancourt's ex-husband, Fabrice Delloye, said on France's BFM-TV Thursday. "Then we hope the emissaries will be able to speak with the FARC and consider pulling Ingrid out of the jungle."

Delloye said Betancourt is believed to be in the region around the southern Colombian city of San Jose de Guaviare or possibly in an adjacent area. Six other hostages were freed from the region earlier this year.

"A humanitarian mission of three facilitator countries, Spain, France and Switzerland, has started, in liaison with concerned authorities," Sarkozy's office said in a brief statement Wednesday.

Betancourt was kidnapped by the FARC in 2002 while campaigning in rural Colombia. The guerrillas have said they want to exchange Betancourt, as well as dozens of other hostages, for hundreds of rebels being held in government prisons. But years have passed with the rebels and Colombia's government failing to agree to a swap.

The guerrilla group has been quiet about a possible prisoner swap since March 1, when Colombian troops killed their chief spokesman and 24 others in a bombing raid in neighboring Ecuador.

On Tuesday, Sarkozy appealed directly to the leader of FARC, Manuel Marulanda, for Betancourt's freedom, saying that without proper care her death was "imminent." His appeal was televised and subtitled in Spanish.

Betancourt's son, Lorenzo Delloye, told reporters Wednesday that his mother suffers from hepatitis B and a skin disease that necessitate a blood transfusion "in the coming hours" or she could lose her life.

The son said he had received the information about his mother's ill health from a former FARC hostage, Luis Eladio Perez, who spent part of his captivity with Betancourt.

Colombia has agreed to allow the mission to try to access the jungle hideaway where she is being held, and pledged to suspend military operations in any area where the envoys might travel.

French authorities have extended an offer to welcome rebel prisoners freed by Colombia in any eventual prisoner-hostage swap deal for Betancourt.