Fidel (L) and Raul Castro. AFP FILE PHOTO

The government released the first news on Fidel Castro's health in two days and sought to assert Raul's leadership four days after the brothers' temporary handover of power.

But Cubans had yet to see the siblings since the unprecedented transition, which has cast a cloud of uncertainty over a country that had been ruled by Fidel Castro without interruption since 1959.

"Fidel underwent surgery from which he is recovering satisfactorily," Health Minister Jose Ramon Balaguer said during a visit to Guatemala, refusing to say more about Castro's health following an operation to stem intestinal bleeding.

The comments from Balaguer, who is considered an influential Communist party official, came two days after National Assembly speaker Ricardo Alarcon said Castro, 79, appeared "very alert."

Earlier, the ruling Communist Party newspaper, Granma, said Cuba was prepared to defend itself but remained calm, as the United States urged Cubans to press for democracy and some exiled Cubans called on those at home to revolt.

"We are prepared for the defense ... and Raul is firmly at the helm of the nation," the official newspaper said.

Since the transition, Cuba was placed under heightened alert with reservists called to military duty and neighborhood watch groups prepared to defend the country of more than 11 million.

Meanwhile its arch-rival, the United States, stepped up calls for a democratic transition in Havana.

"All Cubans who desire peaceful democratic change can count on the support of the United States," US Secretary of State Condoleezza Rice said in a message carried on Radio and TV Marti, a US-funded broadcasting operation designed to send uncensored news to the people of Cuba.

"The United States is also encouraging all democratic nations to join together and call for the release of political prisoners, for the restoration of your fundamental freedoms, and for a transition that quickly leads to multiparty elections in Cuba," Rice said.

It is unclear how many Cubans actually heard her message, as Cuba goes to great lengths to block Radio and TV Marti transmissions.

The White House also dismissed Cuba's fears of a US invasion as "absurd."

 "The US has absolutely no designs on invading Cuba," White House spokesman Tony Snow told reporters in Washington.

Cuban exiles in Miami applauded President George W. Bush's first statement on the power handover, issued Thursday, in which the US leader urged Cubans to "work for democratic change" and pledged Washington's support in their effort.

 "We support the remarks 100 percent," Camila Ruiz, spokeswoman for the Cuban American National Foundation, which has called for an uprising in Cuba, told AFP. "This is just the beginning."

 Cuban state-run media dismissed Bush's call for democracy as "inadmissible" and "the epitome of delirium."

 "In Cuba, calm reigns, but in the United States, and in particular Miami, a little extreme-right group is aching," Granma said.

The absence of Raul and Fidel Castro has fueled speculation about the transition and the elder brother's condition.

The centre-left daily El Pais newspaper from Spain reported that Raul Castro has appointed a team of six senior communists to govern under his supervision.

Raul Castro's ruling team reportedly includes Cuba's Foreign Minister Felipe Perez Roque and Carlos Lage, who championed the country's economic reforms in the 1990s.

Also on board are three more traditional communists: Ramon Machado Ventura, Jose Ramon Balaguer and Esteban Lazo, El Pais said.

The sixth member is the president of Cuba's central bank, Francisco Soberon, who favours a centralized approach to economic policies.

The Barcelona daily La Vanguardia, citing diplomatic sources, said that Raul Castro had not appeared in public since taking power because he was suffering from "depression" due to the serious illness of his wife Vilma Espin.

Fidel Castro, who turns 80 on August 13, has last been heard from in a statement attributed to him late Tuesday, saying he was in "good spirits" and that the armed forces were ready to defend the country. Castro was last seen publicly on July 26.