(Updates with Richardson, street protests, son's comments)

By Patrick Markey

BOGOTA, April 4 (Reuters) - A French medical mission to treat ailing Colombian rebel hostage Ingrid Betancourt remained grounded at a Bogota air base on Friday, dogged by doubts after guerrillas rejected calls for her swift release.

Betancourt, a French-Colombian citizen and former presidential candidate who was kidnapped while campaigning in 2002, is the highest profile hostage being held by the Revolutionary Armed Forces of Colombia, or FARC.

A French aircraft carrying a medical team arrived on Thursday and sat on the tarmac at Catam military base waiting to fly into the jungle, where former hostages say the mother of two is very sick with hepatitis and other illnesses.

French Foreign Minister Bernard Kouchner acknowledged France was waiting for a response from rebels, and her sister was unsure the medical team could reach her without a prior deal with the FARC.

"Are we pessimistic about a result from this French mission? Yes," Astrid Betancourt told local radio. "If this is a success in any way, it will be a miracle, as there was no previous agreement."

Waving flags and chanting "Freedom, freedom," several thousand Colombians took to the streets across the country for a midday march to demand the FARC free kidnap victims, some of whom have spent as long as a decade in captivity.

Betancourt's son, Lorenzo Delloye Betancourt, used a news conference in Paris to urge the rebels: "Please make a gesture. Understand that this a historic moment for you."

But a top FARC commander, Rodrigo Granda, said on Thursday Betancourt would not be handed over immediately. He said rebel captives, who include three U.S. contractors and dozens of politicians, police and soldiers, could only be freed through a negotiated agreement to swap hostages for jailed guerrillas.

He did not say whether the medical team would be allowed into the jungle camps.

The International Committee of the Red Cross, which has helped in previous hostage initiatives, says it has had no contact with rebels over the medical mission and is not engaged in the French effort.

'WE'RE TRYING, TRYING'

Kouchner said France kept working for the mission and told reporters that President Nicolas Sarkozy, who has made Betancourt's release a priority, would travel to meet Venezuelan President Hugo Chavez if it helped free her.

"We're trying, trying, trying and there's no other solution," he told Europe 1 Radio.

Chavez, a left-wing critic of Washington, has helped broker the release of six FARC hostages this year. His involvement and demands for political recognition for the Marxist rebels have fueled tensions with neighboring Colombia, a White House ally.

Captives freed recently have described Betancourt as being in very poor health and say she has been chained up after several escape attempts. A rebel video shown last year showed her gaunt and despondent in a jungle camp.

Violence from Colombia's war has eased as President Alvaro Uribe's U.S.-backed troops push rebels into remote areas. The FARC has an estimated 9,0000 guerrillas but is labeled a cocaine-trafficking terrorist group by U.S. and European Union officials.

The FARC says it wants to swap 40 hostages, including Betancourt and the three Americans captured in 2003, for jailed rebels. Attempts to reach a deal are deadlocked over a FARC demand that Uribe pull back troops from a swath of land.

New Mexico Gov. Bill Richardson, a seasoned negotiator, will visit Venezuela next month to discuss the three American defense contractors. He was in Colombia last week.

The three were captured when their plane went down in the Colombian jungle in February 2003; the FARC has said they must be swapped for two rebel members jailed in the United States.

The death of two top FARC commanders in Colombian military raids in March has complicated talks. One commander, Raul Reyes, was a French contact in negotiating a hostage accord. (Additional reporting by James MacKenzie in Paris; Editing by Doina Chiacu)

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