US President George W Bush has urged Cubans to work for democratic change in his first public comments since Fidel Castro had stomach surgery on Monday.
Life in communist Havana ostensibly continues as normal
He pledged Washington's support for Cubans who sought to "build a transitional government in Cuba committed to democracy".
The US blockade of Cuba began in 1962, three years after Mr Castro took power.
Since Mr Castro passed power to his brother Raul, three days ago, no images have been released of either man.
Street interviews with the members of the Cuban public have been repeatedly broadcast on Cuban government-controlled television.
The interviewees have been wishing Fidel Castro a speedy recovery, and voicing their total confidence in Raul Castro, who is currently Cuba's acting president.
Some Cubans are beginning to wonder what might be going on behind the scenes here, reports the BBC's Stephen Gibbs in Havana.
The question of Raul Castro's non-appearance remains unanswered.
Inevitably, there are also plenty of rumours going about as to the real state of Fidel Castro's health, our correspondent says.
In a written statement issued on his way to his summer retreat in Texas, Mr Bush also warned that the US would "take note of those, in the current Cuban regime, who obstruct [Cubans'] desire for a free Cuba".
The US is home to a large Cuban exile community based in Miami, much of which is hostile to the communist authorities in their home country.
Cuban media have been stressing that the armed forces are ready for any attack on the communist system.
Earlier, the US state department criticised the "imposition" of Raul Castro as acting leader of Cuba.
The move "denies the Cuban people of their right to freely elect their government," spokesman Sean McCormack said.
He added that Washington stood ready to support any "genuine transition" with humanitarian relief.
'Stop the hate'
Juanita Castro - estranged younger sister of the ailing Cuban leader - has hit out at the celebrations that have been taking place in Miami following news of his ill-health.
Ms Castro, who has been in exile in the US since the 1960s, told the BBC from Miami that she thought images of celebrations in the city were damaging "the cause, the country and the exiles".
It was time to stop the hate, she said.
She also said she had heard from sources close to her brother that he was out of intensive care and in a stable condition.
Some people in Miami believe the Cuban leader is dead.
The Cuban leader, who turns 80 this month, has been quoted as saying that a punishing schedule in recent weeks had affected his health.
It is not clear whether he is in hospital or recovering at home.
This is the first time Mr Castro has relinquished any of his duties as head of the communist state since he came to power in 1959.
Fidel Castro has been among the world's longest-ruling leaders, outlasting nine US presidents.