Miami/Washington  - For decades, the Cuba of Fidel and Raul Castro has accused the CIA of doing all it could to destabilize the the communist island.

Now, Juanita Castro, a rebellious sister of the men who have led Cuba for over half-a-century, has revealed that those attempts reached even into the Castro family itself. She was herself a CIA agent from 1961 until she left Cuba in 1964, she admitted.

At 76, a resident of Miami for 45 years, the fifth-oldest of the seven Castro siblings revealed her "great secret" in a television programme late Sunday. Even her famous brothers did not know that she had worked for the CIA for three years prior to leaving the island, she said.

"The CIA - they wanted to talk to me because they had interesting things to tell me and interesting things to ask me. Whether I was willing to run that risk, whether I was willing to listen to them. I was left in a bit of a shock, but I still said yes," Juanita Castro recalled.

Her confession is the focus of over 400 pages of her memoirs, "Fidel and Raul, My Brothers: The Secret History," which went on sale Monday in the United States, Mexico, Spain and Colombia.

To keep up the suspense, Miami television channel Canal 23 broadcast late Sunday only the first of several programmes in which Juanita Castro is to share her story in the coming weeks.

The Miami daily El Nuevo Herald, which had advance access to the book, reported Monday that Juanita Castro was agent "Donna" for the CIA. She claims she was initially recruited for the job by the wife of Vasco Leitao Da Cunha, Brazil's ambassador in Havana at the time and later Brazil's foreign minister.

Her CIA contact was agent "Enrique," the alias hiding the identity of Tony Sforza. Juanita Castro described him as a "key element" in the Cuban Project, the failed economic sabotage activities that US intelligence promoted in Cuba following the Bay of Pigs fiasco - a failed invasion of Cuba by CIA-trained force of Cuban exiles - in 1961.

A year later, the standoff over Soviet missiles stationed on the communist Caribbean island just 160 kilometres off the US coast risked setting off World War III.

El Nuevo Herald stressed that Juanita Castro claimed to have passed on information to the CIA through the Brazilian ambassador's wife Virginia Leitao da Cunha that Soviet rockets were being installed in Cuba and that there were growing numbers of Russian personnel on the island.

The woman's first contact with the CIA took place at the Camino Real Hotel in Mexico City, where she travelled with the excuse of visiting her sister Enma, the newspaper said.

According to her memoirs, however, Juanita Castro cooperated with the CIA on condition of "not taking part in any plotting tasks aimed at the physical elimination of her brothers."

She did not have to wait long for her first mission: the transport of food tins containing messages and money for CIA agents in Cuba, the newspaper reported citing the book.

Juanita Castro used coded messages through a short-wave radio to stay in touch with the CIA.

She said she chose to risk her own life by cooperating with the United States to save the lives of her compatriots.

Based on the book - whose launch was shrouded in secrecy to prevent leaks of details - El Nuevo Herald wrote that the CIA decided to take Juanita Castro out of Cuba after current Cuban President Raul Castro visited her to tell her she was being investigated for "counter-revolutionary activities."

"The last time I spoke to my brother Raul was on June 18, 1964, the day before I left. That was the last time I spoke to him or saw him personally," she said in the interview that was broadcast late Sunday.

Although the story caused quite a stir in Miami - the stronghold of anti-Castro feeling in the United States - it did not immediately have a major impact in the rest of the country.

Cuba claims that the CIA made more than 600 attempts to kill Fidel Castro. At least some of them were officially confirmed in 2007, when the CIA declassified secret documents known as the "family jewels," which acknowledge that in the early 1960s - precisely the time when Juanita Castro worked as agent "Donna" - there were several assassination attempts.

The documents showed that then US Attorney General Robert Kennedy personally moved to secure the assassination of Fidel Castro.

Starting in 1973, Juanita Castro owned a chemist's store in Miami, which she sold two years ago.

"Juanita owed us all this portion of history and she has been very brave to decide to tell this secret," Mexican journalist Maria Antonieta Collins, who wrote the book with Juanita Castro, was quoted as saying. (dpa)