Vladimir Morera was among a group of 53 prisoners freed when the United States and Cuba announced their historic rapprochement last year, but he was re-arrested in April and has been on hunger strike for 80 days to protest his detention, according to his family.
Relatives and fellow dissidents opposed to the communist regime told AFP Morera had been transferred to hospital because of deteriorating health.
"He's in grave condition. We saw him five days ago. But now they say he's a prisoner and won't allow us to visit him," said his sister Cristina.
"His life is in danger. No dissidents have been allowed to see him, but we know from his family that he is in critical condition. The government doesn't want anyone to know about his case," said leading dissident Guillermo Farinas.
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The US yesterday "urgently" called on Cuban President Raul Castro's government to free Morera.
"The United States is deeply concerned about (his) deteriorating physical condition," US State Department spokesman Mark Toner told journalists.
The Cuban government, which denies holding political prisoners, has not commented on Morera's case.
Fellow dissidents say he was in fact the one who was assaulted by regime supporters who stormed his house after he boycotted Cuba's local elections, which dissident groups condemn as undemocratic.
The release of Morera and dozens of other prisoners was a cornerstone of the US-Cuban rapprochement announced on December 17, 2014, which opened a new chapter in the countries' relations after half a century of Cold War enmity.