The United States is calling for the immediate release of a prominent Vietnamese dissident who has been taken back into custody despite frail health.

A U.S. State Department spokeswoman issued a statement late Tuesday expressing “concern” over the re-arrest of Catholic priest Nguyen Van Ly.

Father Ly has been a long-time critic of Vietnam's ruling communist government, and was sentenced to eight years in prison in 2007 for his pro-democracy activism. He was granted medical parole last year to receive treatment for a brain tumor and put under house arrest.

Vietnamese state media say the priest was re-arrested Monday because he continued his anti-government activities, such as criticizing the government's response to a maritime dispute with China.

The State Department said no one should be imprisoned for exercising the right to free speech.

Ly's return to detention was also criticized Wednesday by the international advocacy group Human Rights Watch. Phil Robinson, the group's Asia director, said the latest arrest “only compounds the cruelty and injustice of his original sentence.”

According to both Human Rights Watch and his family, Ly has suffered three strokes since 2009. A bipartisan group of U.S. lawmakers urged the Vietnamese government not to return him to prison when a medical parole expired in March.

Father Ly has spent more than 16 years in jail since the 1970s. He is one of the co-founders of the pro-democracy movement known as “Bloc 8406.”