Social media sites and some news organizations have reported allegations by a Venezuelan doctor that Castro, 86, suffered a massive stroke, was in a vegetative state and had only weeks to live, though the same doctor, Jose Rafael Marquina, has made some claims before that have not panned out.
Marquina told the newspaper ABC in Spain that Castro had suffered a "massive embolism of the right cerebral artery" and while not on life support or breathing artificially, was "moribund" at a house in a gated former country club in western Havana.
Marquina also said Venezuelan President Hugo Chavez had traveled suddenly to Havana to be with his friend and ally, an account that could not be immediately verified.
Reached by The Associated Press, Marquina said his sources were in Venezuela, but he would not identify them or say how they were in a position to have information about Castro's health. He also indicated he had received corroborating evidence from sources on Twitter, but would not say who.
In April, Marquina said that Chavez, who has been battling an undisclosed kind of cancer, was in his "last days" and would not last to November. With less than two weeks to go, the Venezuelan leader says he's beaten the illness and appears stronger in public.
Castro's health is considered a matter of national security in Cuba and few details are released.
Rumors that the former Cuban leader has died or is near death have circulated repeatedly for years, but they gained force after he failed to issue a public statement congratulating Chavez on his Oct. 7 election victory.
Castro has not been seen in public since March, when he received visiting Pope Benedict XVI. He has also stopped writing his once-constant opinion pieces, the last of which appeared in June.
There was no immediate comment from the Cuban government on the latest claims, but a letter attributed to Castro was published Thursday by Cuban state media. In it, he congratulated graduates of a medical school on the occasion of its 50th anniversary.
Two close family members of Castro have also recently denied he is in grave condition. Juanita Castro, the former leader's sister, told the AP in Miami that reports of her brother's condition are "pure rumors" and "absurd."
Son Alex Castro told a reporter for a weekly Cuban newspaper that his father "is well, going about his daily life."