MIAMI (Reuters) - Using the code name Donna, the younger sister of Fidel and Raul Castro worked undercover for the CIA in Cuba in the early 1960s, helping opponents of their communist rule escape execution and imprisonment, she said in memoirs published in exile on Monday.

A photograph of Cuba's President Fidel Castro hangs in between buildings in Havana, August 6, 2006. REUTERS/Enrique De La Osa

Revealing what the publishers called a closely guarded secret kept hidden for four decades, Juanita Castro described in the book how she was recruited by the U.S. Central Intelligence Agency in Havana two years after the 1959 Revolution led by her brothers, which she initially supported.

There was no immediate reaction to her revelation from the U.S. authorities or the Cuban government, which routinely dismisses critics as mercenaries in the pay of Washington.

Juanita Castro, 76, broke publicly with the Cuban government led by her brother Fidel Castro in 1964 after leaving Cuba for Mexico. She went into exile in Miami and has remained a firm critic of communist rule in Cuba.

In the memoirs entitled “Fidel and Raul, My Brothers, the Secret History,” told to Mexican journalist Maria Antonieta Collins, she says she quickly became disenchanted with Fidel Castro’s rule over the Caribbean’s largest island because he increasingly persecuted opponents and turned to communism.

She says Fidel Castro “betrayed” her and other Cubans by abandoning the nationalist democratic revolution he had promised and imposing a one-party Marxist state on Cuba.

Juanita Castro wrote she was recruited to be a clandestine CIA operative by her friend Virginia Leitao da Cunha, the wife of the Brazilian ambassador to Cuba, who in 1958 sheltered her and other revolutionary followers of Castro during the armed struggle to topple dictator Fulgencio Batista.

She said that at a meeting with an American CIA officer “Enrique” in a Mexico City hotel in 1961, she was given the code name Donna and codebooks to use in Cuba with a short-wave radio to receive instructions from her CIA handlers.

Former leader Fidel Castro, 83, who last year handed over the presidency of Cuba to his younger brother Raul, 78, for health reasons, has long considered the CIA his arch-enemy. He says the U.S. spy agency was behind most of the 600 or so assassination plots he claims were made against his life.

In her memoirs, Juanita Castro wrote she agreed to work for the CIA under the noses of her brothers on the condition that she was not asked to participate in any violent acts against them or any other member of their government.

“Did I feel remorse about betraying Fidel by agreeing to meet with his enemies? No, for one simple reason: I didn’t betray him. He betrayed me,” she writes in the 432-page book published in Spanish by Grupo Santillana.

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“He betrayed the thousands of us who suffered and fought for the revolution that he had offered, one that was generous and just and would bring peace and democracy to Cuba, and which, as he himself had promised, would be as ‘Cuban as palm trees,’” she said.

Juanita Castro said in her memoirs and in a TV interview that she never accepted any money from the CIA for her collaboration. “I never put any price on my desertion ... on my activities against the communist dictatorship,” she told the Spanish language TV channel Univision-Noticias 23 on Monday.

She described in her book how, following CIA instructions often secretly picked up at isolated roadside drop points in Cuba, she helped people persecuted by Fidel Castro’s secret police to escape capture, imprisonment and possible execution.

Some were sheltered at the house where she lived with her mother, Lina Ruz Gonzalez, who was also the mother of Fidel and Raul Castro. Lina Ruz, who also helped some friends escape persecution, died in 1963, Juanita Castro said.

She recalled her own shock when Fidel Castro, who had denied publicly that he was a communist, declared on December 2, 1961, that he was a Marxist-Leninist and that he would remain one for the rest of his life.

“Fidel’s radical change to communism was not out of political conviction, but simply out of the need to hold power, which is what has always been important to him,” she wrote.

“I have no other explanation: He turned to the Soviet Union to perpetuate himself in power.”

The collapse in 1991 of the Soviet Union, Cuba’s main ally and economic benefactor for years, plunged the island into economic crisis. But despite the economic problems, both Fidel and Raul Castro have ruled out any shift to capitalism.

In response to a call for a “new beginning” in U.S.-Cuban ties made by U.S. President Barack Obama, the Cuban leadership has started talks with Washington on issues like migration and postal service ties, but it demands that Obama completely end the 47-year-old U.S. trade embargo against Cuba.

Obama says he wants to see Havana free jailed dissidents and improve human rights.

Juanita Castro says she has not spoken to Fidel or Raul Castro since she left Cuba in 1964.