"In general, his state of health is increasingly deteriorating," Nguyen Thi Hieu said of her brother, Nguyen Van Ly.

Ly's conviction and eight-year sentence in 2007 drew condemnation from diplomats, Vietnam watchers and human rights groups.

Prosecutors said Ly was a founding member of the banned "Bloc 8406" pro-democracy coalition, and that he was also a driving force behind the outlawed Vietnam Progression Party.

In December, his sister said Ly, who is in his early 60s, had been transferred back to prison after almost one month in hospital because of high blood pressure that led to a stroke.

"He has problems walking with his right leg, and suffers from hypertension" which prevents him from being able to write, Ly's sister said on Tuesday.

"Our family has to regularly send medicine to him."

Global human rights watchdog Amnesty International in December called on Vietnam to release the priest to his family "so that they can ensure he receives the proper medical care, including hospitalisation, that he needs."

In July last year a bipartisan group of 37 United States senators sent a letter to Vietnamese President Nguyen Minh Triet calling for Ly's release.

The United States embassy last year also expressed concern over his imprisonment.

Ly's return to prison in December came on the same day that Triet held a rare meeting with Pope Benedict XVI at the Holy See. Both sides hailed that meeting as a prelude to improved ties.

Vietnam and the Vatican do not have diplomatic relations but in recent years have begun a reconciliation.

It was not clear whether Ly's situation was discussed at their talks.

Religious activity remains under state control in Vietnam but the government says it always respects the freedom of belief and religion.

Analysts, rights groups and diplomats say the human rights situation in Vietnam has been worsening.

Agence France Presse - February 9, 2010