BOGOTA, Colombia — A senior rebel leader ruled out the unilateral release of an ailing French-Colombian hostage, casting doubt on the mission of French diplomats and doctors who flew to Colombia yesterday to save her life.

The rebels’ foreign relations chief, Rodrigo Granda, did not mention the French plane sitting on the tarmac in Bogota, but suggested the French had no deal with the rebels for the release of Ingrid Betancourt, who was kidnapped while running for president of Colombia six years ago. “Only as a result of a prisoner exchange will those who are held captive in our camps go free,” Mr. Granda said in a statement posted yesterday on a Web site.

Ms. Betancourt’s plight has taken on added urgency since another hostage who spent months with her was released in February, telling the Associated Press she has hepatitis B and a tropical skin ailment. Colombian news press and broadcast outlets have reported this week that she is at death’s door, giving few details and citing unidentified peasants who say they have seen her.