The men are celebrated by many in Cuba as national heroes and said they were monitoring armed exile groups in Miami to prevent attacks on the Caribbean island.

The US regularly calls for the release of political prisoners held in Cuban jails.

Diplomatic thaw

Obama has raised hopes of thaw in diplomatic ties severed for more than 40 years, after saying he would consider he was open to holding talks.

But he has said he would keep the US trade embargo, in place since 1962, as leverage to influence the one-party state.

On Tuesday, leaders attending the Mercosur summit in the Brazilian resort town of Costa Do Sauipe also issued a special statement demanding an end to the US embargo.

On Monday, Castro told Al Jazeera that the US must make concessions first if the two countries are ever to restore diplomatic ties severed for more than 40 years.