Father Ly is a strong advocate for democracy in Vietnam
A group of 37 United States senators has written to Vietnam's president calling for the "immediate and unconditional release" of a priest.
Father Nguyen Van Ly was jailed for eight years in 2007 for spreading propaganda against the communist state.
The senators believe the trial in the central city of Hue, which lasted just half a day, was seriously flawed.
The 63-year old priest has been jailed three times since the 1970s for a total of 17 years.
The senators - led by Democrat Barbara Boxer and Republican Sam Brownback - asked Vietnam's President Nguyen Minh Triet to intervene in the case.
Their letter said Ly's trial appeared "seriously flawed," noting that the pro-democracy activist was denied access to counsel and was prevented from presenting a defence.
"Given these serious flaws in relation to his arrest, trial and imprisonment, we request that you facilitate Father Ly's immediate and unconditional release from prison, and allow him to return to his home and work without restrictions on his right to freedom of expression, association and movement," the letter said.
The senators' letter reminded the president of Vietnam's commitment to protect the rights of criminal defendants.
"Father Ly's arrest, trial and ongoing detention in this instance call into question Vietnam's commitment to these fundamental principles," wrote the lawmakers.
They also asked about Father Ly's "health and welfare".
It follows the recent arrest of the Vietnamese human rights lawyer, Le Cong Dinh, who was arrested for "propaganda" against the state.
Father Ly was a founding member of Bloc 8406, a pro-democracy movement launched in 2006.