The eldest son of Cuban leader Fidel Castro said on Monday (November 13) his father is recovering from an undisclosed illness that has forced him to temporarily give up power he held since a 1959 revolution. Fidel Castro Diaz-Balart, better known in Cuba as "Fidelito," did not answer when asked if his father would be well enough to attend a December 2 public celebration of his 80th birthday. "Here it has been said officially, he even appeared in a video two weeks where he explained in his own words his state of health," the younger Castro told Reuters when asked about the condition of the Cuban leader, who has not been seen in public since undergoing intestinal surgery in late July. "The only thing I can tell you is that he is recovering. Many thanks," he said before quickly walking away. "Fidelito" Castro, who is 57 and a nuclear physicist, spoke to Reuters at a biotechnology conference in Havana. Father Fidel Castro has appeared only in photos and videos since his surgery and in the latest, released on October 28, he looked to be in such bad health that it raised questions about his future. U.S. officials in Washington told Reuters on Monday that the "working assumption" is that Castro has some form of cancer, but that it has not been confirmed. He turned 80 on August 13, but he was not well enough to take part in a public celebration and said it should be put off until December 2, when Cuba celebrates the start of the revolution that put him power. Recently, Cuban officials have said it was not known if Castro will be able to attend the event, saying it depends on his condition. His brother Raul, who has been defence minister since 1959, has been in charge of the government since Castro's surgery, announced on July 31.