Castro, 79, temporarily handed over power to his younger brother Raul on Monday after he underwent surgery for gastrointestinal bleeding.
On a visit to Guatemala on Friday, Jose Ramon Balaguer, the health minister, said the veteran leader was recovering well from the operation.
"We know Comandante Fidel will recover soon and will be back with us soon."
Also on Friday, Condoleezza Rice, the US secretary of state, appealed directly to Cubans to stay at home and strive for democracy.
"All Cubans who desire peaceful democratic change can count on the support of the United States," Rice said in a message intended for broadcast via networks that reach the Cuban people.
"We encourage the Cuban people to work at home for positive change, and we stand ready to provide you with humanitarian assistance as you begin to chart a new course for your country," she added.
Cuba's health minister says
Castro is recovering well
The US is anxious to avoid a flood of refugees caused by the political uncertainty that has followed Castro's illness, and Rice made clear Cubans should not attempt to cross the Florida Straits to reach the US.
"Clearly we believe that Cubans should stay in Cuba and be a part of what will be a transition to democracy."
Referring to the handover of power to Raul Castro, Rice said it was unacceptable to "go from one dictator to another".
Neither Castro has been seen in public since the announcement of the elder brother's surgery, and state media in Cuba moved to quell rumours of a power vacuum on Friday.
The Communist newspaper Granma said Raul was "firmly at the helm" of the nation and its armed forces.
The paper also rejected earlier calls from George Bush, the US president, for a transition to multi-party democracy.
The Roman Catholic Church, meanwhile, called on Cubans to pray for Castro's recovery.
Cuban paper Granma says Raul
Castro is 'firmly at the helm'
However speculation continues in both Cuba and Miami, home to a large exiled Cuban-American population, over the future as it is the first time since his revolutionary victory in 1959 that Castro has delegated power to anyone else.
A US government report by the Commission for Assistance to a Free Cuba recommended a month ago that the US act fast to help create a transitional government in Cuba when Castro's rule ends and get advisers on the ground within weeks.
Officials say this plan has not been activated and Tony Snow, the White House spokesman, said on Friday that the US was still trying to assess the situation in Cuba and no changes were planned in overall policy at this point.
Snow also sought to reassure Cubans the US would not use the uncertainty surrounding Castro's illness to invade the communist island.