Castro (80) has not been seen in public since undergoing surgery in July and handing over political power.

Mr Chavez spoke at the Latin American summit in Brazil in a rare admission of the state of Castro's health.

Venezuela's president said he spoke to the Cuban leader by telephone for half an hour several days ago.

"He is going through a difficult situation but just like he says, the machine that they have to fix is 80 years old, " Chavez told the Mercosaur trade summit.

"I cannot give details [of his health] because I am not his doctor, and even if I was I would not give them to you.

"He's back in the Sierra Maestra and locked in a battle for his life, " Chavez said.

The Sierra Maestra is a rugged mountain range in south-east Cuba with a long history of guerrilla warfare.

After Fidel Castro returned to Cuba from exile in Mexico, he and his fellow rebels hid out in the Sierra Maestra from where they were able to expand their 26 July Movement, building up their guerrilla forces and starting the revolution which in 1959 overthrew the regime led by Fulgencio Batista.

"There are those who want Fidel to die. . . [but] we have confidence he will recover completely. I don't know when Fidel will die, I hope he lives 80 more years, I hope he lives 100 more years."

There have been steady reports suggesting that Castro's condition is extremely serious. Dr Jose Luis Garcia Sabrido, head of surgery at Madrid's Gregorio Maranon public hospital, described Mr Castro's recovery as "slow but progressive" after examining the Cuban leader last year.

Earlier this week a Spanish newspaper quoted medical sources as saying Mr Castro opted for a risky medical procedure that led to grave complications.