WASHINGTON/HAVANA (Reuters) - The United States on Tuesday urged Cuba to free two men who Amnesty International has called “prisoners of conscience,” saying their imprisonment exemplifies how the Cuban government continues to silence its citizens’ peaceful opposition.

Human rights activist Eduardo Cardet is serving a three-year sentence for allegedly criticizing former Cuban leader Fidel Castro, the U.S. State Department said, while environmental activist Ariel Ruiz Urquiola is serving a year in jail for allegedly disrespecting government authority.

“We call on the government of Cuba to release all political prisoners immediately,” the U.S. State Department said, adding that it was “deeply troubled” by reports that Urquiola was on hunger strike and in a critical medical condition.

Cuba did not immediately respond to a request for comment. Officially, the government maintains it does not have any political prisoners, and characterizes Cuba’s small but vocal dissident community as mercenaries paid by U.S. interests to destabilize the government.

Urquiola’s sister, Omara Ruiz Urquiola, told Reuters that her brother had been taken to a hospital last Friday after embarking on a hunger and thirst strike.

“He is very weak because he does not accept his penitentiary condition,” she said, adding that local authorities had not allowed her to see him.

The United States criticizes Cuba’s one-party system and repression of free speech and assembly, while Cuba criticizes its northern neighbor for racial disparities, lack of universal health care, and currently for its immigration policies.