Havana/Caracas - The health of former Cuban leader and prominent revolutionary Fidel Castro has worsened, according to reports circulating on Saturday.

The reports, predominantly on the microblogging network Twitter, appeared to have been sparked by comments made late Friday by Venezuelan doctor Jose Rafael Marquina to Spanish newspaper ABC.

Marquina was reported as saying Castro, 86, had suffered a "massive embolism" to an artery in the right side of his brain.

He was not on life support or mechanical breathing, "but his health is precarious," the doctor said, while declining to reveal his sources.

Castro's son Alex on October 11 denied earlier reports of his father's failing health.

"El Comandante is well," he was quoted as saying by a Cuban blogger who goes by the name of Yohandry Fontana, using his father's nom de guerre and political nickname.

" (He is ) going about his daily business, reading, doing his exercises," the younger Castro said.

On Thursday, Public Health Minister Roberto Morales read out loud a message reportedly written by Fidel Castro earlier that day, to congratulate students of a medical training facility on their graduation.

Castro has not been seen in public since March, when he met with the visiting Pope Benedict XVI. His newspaper column "Fidel's Reflections" stopped appearing in June.

The former Cuban leader also failed publicly to congratulate the success of his protege Hugo Chavez in Venezuela's presidential elections this month.

Earlier pronouncements by Marquina have caused the state media in his native Venezuela to label him a "charlatan." In April he predicted that Chavez, who has undergone treatment for cancer, was living his "final days."