High Commissioner for Peace Luis Carlos Restrepo said the tapes were confiscated after the arrest Thursday evening of three suspected urban members of the Revolutionary Armed Forces of Colombia, better known as the FARC.
The tapes are the first images in years providing evidence the hostages may be alive.
The videotapes, which were played at a news conference without sound, showed an extremely gaunt Betancourt, apparently chained, in front a jungle backdrop. In the silent images, Betancourt, who was kidnapped in 2002, has long hair and stares blankly at the ground.
The government said that the tapes carried the date for Betancourt of Oct. 24, 2007. The tape of the Americans carried the date of Jan. 1, 2007, but a kidnapped Colombian soldier, who appeared on the same tape, said the recording was being made on Oct. 23.
The Americans, Thomas Howes, Keith Stansell and Marc Gonsalves, were abducted by the FARC after their plane went down in southern Colombia in 2003.
Restrepo said the five tapes showed images of 12 other Colombians, mainly members of the security forces.
The FARC are offering to release these and other hostages in exchange for the freeing of hundreds of rebels from Colombian and U.S. prisons.
Venezuelan leader Hugo Chavez had been involved in trying to mediate a deal between the government and the rebels until last week when President Alvaro Uribe ended his role, saying Chavez had disobeyed a direct order in contacting the head of Colombia's army.
During his time as mediator, Chavez had demanded from the FARC so-called "proof of life" of the hostages, evidence that they were indeed still alive.
Restrepo said the army also found various letters, including an undated letter from Howes to his wife, as well as a will dated Nov. 26, 2006. A letter from Gonsalves to the military commander of the FARC, known as "Mono Jojoy", dated Oct. 23, 2007, and a letter from Betancourt to her mother dated Oct. 24, 2007.