"We are very concerned by the denial of activist Cu Huy Ha Vu's appeal," State Department spokesman Mark Toner told reporters.

"We continue to urge the government of Vietnam to immediately release Vu as well as all other prisoners of conscience and believe that no individual should be imprisoned for exercising the right to free speech," Toner said.

Vu, the son of a revolutionary leader, was detained in November after twice trying to sue prime minister Nguyen Tan Dung over a controversial bauxite mining plan and a decree that prevented class-action petitions.

Vu was officially accused of spreading propaganda against the state. But the French-trained legal expert told the appeal hearing that Dung pursued the case after the attempted lawsuits caused the prime minister to lose face.

The United States has a growing relationship with Vietnam and just Monday announced the start of the first regular military cooperation with its former woe foe. Vietnam has been eager to build ties amid high tension with historic rival China.

A number of members of the US Congress have voiced alarm about the eagerness to build relations with Vietnam, saying that President Barack Obama's administration should condition assistance on improvements in human rights.

Toner promised that the administration would keep raising human rights.

"Our bilateral relationship with Vietnam is in general very good, but we do have serious concerns about human rights issues and we're going to continue to press that," he said.

Vietnam last week re-arrested Nguyen Van Ly, a Catholic priest and leading democracy advocate, who had been temporarily released for treatment of a brain tumor.

Agence France Presse - August 2, 2011