"My basic duty is not to cling to office, nor even more so to obstruct the rise of people much younger, but to pass on experiences and ideas whose modest value arises from the exceptional era in which I lived," said Castro, who stepped aside from Cuba's presidency "temporarily" more than 16 months ago after undergoing surgery.

Castro, 81, appeared to hint in his letter that he would leave the country's top leadership to his brother Raul Castro, 76, who has served as interim leader since July 31, 2006.

Castro has not been seen in public other than on television since his surgery, but weekly opinion pieces of his on global affairs have been published in Cuba's newspapers since March.

Questions about his political future arose after he was nominated as a candidate to the National Assembly earlier this month, making him officially eligible to resume the presidency if he should be elected to the assembly in January.

Cuban officials have said Fidel Castro keeps up with official business while he is recovering in an undisclosed location, but there has been no official indication of whether or when he would resume the presidency.

Nor has there been any clear information about the state of his health during his lengthy convalescence.

In his letter, Fidel Castro said he fully concurred with Brazilian architect Oscar Niemeyer, who just turned 100, in that "you have to stick to your principles till the end."

He said he was "deeply convinced" that the problems now facing Cuba require more solutions "than those contained in a chessboard."

"Not even a single detail can be overlooked, and it is never an easy task when human intelligence in a revolutionary society is called to prevail over its instincts."

The letter, which was shown to bear his signature on television, rekindled speculation on Fidel's political future, with some Cubans believing it signaled his political demise, while for others it suggested he would continue in power until the bitter end.

Fidel Castro also commented on Saturday's accord in Bali on a 2009 deadline for negotiations on a new treaty to tackle global warming, to which the United States -- the world's top emitter of greenhouse gases -- agreed but voiced "serious concerns."

"It is obvious the United States maneuvered to avoid isolation, although it did not signal any change in the empire's somber intentions. The great show started, and Canada and Japan joined it immediately," Castro said.

From the ranks of the nominees for Cuba's national and provincial assemblies, 614 lawmakers will be elected in January and they will choose the Council of State. The council's president serves as head of Cuba's one-party government.

If Fidel had not been nominated to the assembly, that could have opened the way for Raul -- who was also re-nominated -- to formally take over Cuba's presidency next year.

Cuba-watchers say it is possible Fidel Castro might be elected an assembly deputy, but then choose not to run for re-election to the Council of State.

Voting for the presidency is set to be held no later than March 5, 2008.

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