Havana (AFP) - Fidel Castro, the towering icon of Cuba's five-decade standoff with the United States, was conspicuous in his absence as his brother and successor Raul sealed a historic detente with Washington.

The father of the Cuban Revolution has been invisible in the media since Wednesday's announcement that the two countries would renew diplomatic ties -- unusual for this nation used to more than 50 years of his colossal presence on the airwaves.

Even since handing power to Raul eight years ago amid a health crisis, Fidel has loomed large in the nation whose modern history he forged, publishing diatribes against US "imperialism" in the state-run press and welcoming the world's leftist leaders to his home.

Political analysts and diplomats said "El Comandante," who turned 88 in August, was likely hanging in the background for now because of his declining health, and not because he disagrees with his younger brother's decision.

In 2001, Fidel had vowed that -- whatever it took -- he would bring home the Cuban intelligence agents whose release in a prisoner exchange was at the core of the historic deal.

The return of the spies, who are hailed in Havana as national heroes, and the rapprochement with the US are "Fidel's victory" as well as Raul's, said a Western diplomat.

"But I think unfortunately his health prevents him from making an appearance," said the diplomat, speaking on condition of anonymity.

That assessment was echoed by Gabriel Molina, a veteran journalist in the state-run media and former editor of daily newspaper Granma.

"Fidel can't make an appearance, but this marks the successful conclusion of a diplomatic effort in which he was directly involved, there's no doubt about that," Molina said.

Fidel has made a slow fade into the background since handing power to Raul after major surgery in 2006, a measure at first described as temporary that became permanent in 2008.

His last public appearance was in January, and his "reflections" in the Cuban press have become less and less frequent.

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In July he received Chinese President Xi Jinping and Russia's Vladimir Putin at his home, but without TV cameras present.

A handful of photos are the only visual record of the meetings.

- 'Fidel is irreplaceable' -

Since then, his columns in the state press have shown a subtle shift in tone.

One published on October 14 praised the "great skill" of a New York Times editorial writer who had recently called for the US to end its trade embargo on Cuba.

Another four days later proposed the US and Cuba team up in the fight against Ebola, after the communist island nation sent scores of doctors to West Africa to help contain the epidemic.

A US official said Fidel was not involved in the secret talks on detente that opened in June 2013.

But in Cuba, observers say the rapprochement likely had his approval, since Raul has never shown signs of wanting to supplant his charismatic brother.

"Fidel is irreplaceable," Raul said when he took power in 2006. "All major decisions will be taken in coordination with him."

On Wednesday, Raul cited his brother several times, recalling his 2001 promise to bring home the jailed Cuban spies and insisting the country had not retreated from Fidel's revoluntionary principles.

However, the deal would likely have been impossible under Fidel, said numerous experts and diplomats in Havana.

Across five decades, the "Maximum Leader" built his political identity around his opposition to American "imperialism," a struggle he described as his "destiny."

An icon of the Cold War, he defied 11 American presidents, scores of plots to assassinate him, a trade embargo, the Cuban missile crisis and the disastrous US-backed Bay of Pigs invasion bid.

Raul by contrast has wielded diplomacy, softer rhetoric and tentative steps toward reform to pave the way to a new era in US-Cuban relations that stands to benefit his people.

"Fidel is totally anti-American, while Raul has a more pragmatic view and prioritizes what's useful for the country," a Western diplomat said.