A stronger Mr Castro appeared defiant on a recent video

He was heard speaking live on the daily radio programme of his ally, the Venezuelan President Hugo Chavez.

The 80-year-old leader is believed to be suffering from diverticulitis, a weakening of the walls of the colon.

He said he was "gaining ground", adding he felt he had more "energy and more strength". He was last seen in a video recording released in early February.

Mr Castro's younger brother, Raul, has been acting as Cuban president since July.

'Student again'

"Hello there, illustrious and dear friend, how are you?" Fidel Castro asked President Chavez at the start of an extended conversation broadcast on the Venezuelan leader's Hello President show.

"I feel good and I'm happy," Mr Castro said.

He said his illness had given him more time for reading, joking that he had become a student again.

He did not discuss the question of when, or if, he would be returning to power, instead calling for patience from people expecting daily updates on his condition.

"I can't talk every day. I ask everyone for patience, calm... the country is marching along, which is what is important," he said.

The leaders chatted about current events, with Mr Castro saying Tuesday's stock market slump in China and the US "really proves our ideas".

They ended their conversation proclaiming: "We will prevail."

Hospital visits

Mr Chavez has visited the convalescing Cuban leader several times, most recently in January.

Video of their meetings have been released.

Mr Castro's health is treated as a state secret in Cuba, and has been the subject of much speculation both at home and overseas.

The most regular pronouncements on his health have come from Mr Chavez.

Cuban authorities have denied the claims of US intelligence officials that he has terminal cancer but will only say that Mr Castro is recuperating satisfactorily.

In January, President Chavez denied a report in a Spanish newspaper that said Mr Castro's prognosis was very grave after three failed operations.

A Spanish surgeon who travelled to Cuba at that time to examine Mr Castro also said the report was "without foundation".