Carlos Lage, in Bolivia to attend the country's constitutional assembly, was asked today by reporters when Castro would be back at work. "In a few weeks," he replied.
Meanwhile the US has pledged not to stoke the political crisis in Cuba but warned that Castro, who has defied Washington for decades must not be replaced by a new dictator.
US Secretary of State Condoleezza Rice urged Cubans to stay on the island amid the political tumult but promised Washington would stand by them in a time of political transition.
"We are not going to do anything to stoke a sense of crisis or a sense of instability in Cuba," Ms Rice said on NBC's Meet the Press.
"This is a transition period for the Cuban people, we are going to stand with them for the proposition that there should not simply be the end of one dictatorship and the imposition of another dictatorship."
She also renewed her appeal for Cubans to work for democratic change on the island, rather than leaving en masse for the US as political uncertainty lingers.
Successive US governments have sought unsuccessfully to oust Castro since he came to power in 1959, including an ill-fated invasion backed by the CIA in 1961.
Church services
However, Cuba's Roman Catholic Cardinal Jaime Ortega said after
Sunday mass yesterday: "Never would the church in Cuba support, nor even minimally accept, any foreign invasion."
Before mass, Ortega read a letter from the Catholic Bishop Conference of Cuba, which said: "The bishops of Cuba ask all our communities to pray that God be with President Fidel Castro in his illness and enlighten those who have taken provisional responsibility for the government."
The message, to be read in all churches on the island, said in part: "The delicate health president Fidel Castro Ruz makes for an especially significant moment for our people."
"We entrust to the Virgin of Cobre, patron saint of Cuba, all of our concern at this moment in our nation's history," the letter read.
The letter also invoked "a profound desire for peace and brotherly tolerance among all Cubans, which should not be disturbed by any foreign or domestic situation."
Cuban Americans pray
Many Cuban-Americans spent Sunday praying for their country’s future at the Church of the Little Flower in Coral Gables, Florida.
Talking about Cuba after Castro, Father Arthur Dennison, Pastor of the Church of the Little Flower, said "When the change comes, we all have to work together to form a new society, to form a society based on reconciliation."
Parishioner Sergio Quintana brushed off any talk of Castro's demise as speculation.
But another church goer Pedro Jimenez said with each day that passes with no news of Castro's health "I'm more hopeful, everyday a little bit more".