Cuban President Raul Castro made an unprecedented offer Thursday to exchange political dissidents jailed in his country for five Cubans imprisoned in the U.S. for espionage.

Castro, on his first official visit to Brazil since taking over as president, also reiterated Cuba's willingness to discuss the U.S. embargo with incoming President Barack Obama.

Answering a reporter's question about political prisoners in Cuba, Castro said: "We will send those prisoners you talk about [to the United States] with their families. But give us back our five heroes."

He referred to the so-called "Cuban Five," who were convicted in 2001 on espionage charges and are lionized in Cuba as heroes. Cuban exile groups in the U.S. say they were justly punished.

U.S. President George W. Bush has taken a hard line against Cuba and would not consider such a trade.

Obama has never discussed releasing prisoners and has said he will keep the embargo as leverage until Cuba shows "significant steps toward democracy," starting with freedom for approximately 219 jailed political prisoners.

But he has shown more openness toward the communist island. Obama promised during the campaign that immediately after taking office on Jan. 20, he will lift all restrictions on family travel and cash remittances to Cuba — not just roll them back to previous rules that were tightened by the Bush administration.

The five, arrested in 1998, acknowledged being Cuban agents but said they were not spying on the United States. They said their focus was on U.S.-based exile groups planning "terrorist" actions against the Castro government.

Castro took over as president in February from his ailing brother, Fidel.