Nguyen Quoc Duc Vuong had spoken of his support for democracy in Vietnam and shared news of pro-democracy protests in Hong Kong, according to Human Rights Watch.

He was arrested in September and sentenced Tuesday in a lightning-quick trial, accused of "defaming" Vietnam's leaders.

"We are deeply concerned about the news that activist Nguyen Quoc Duc Vuong was sentenced to eight years in prison," a State Department spokesperson said.

"His lengthy sentence is another in a number of troubling arrests and sentences of journalists, bloggers, and activists aimed at denying freedom of speech in Vietnam," he said.

"We urge the Vietnamese government to ensure its actions and laws, including the Penal Code, are consistent with the human rights provisions of Vietnam's constitution and its international obligations and commitments."

The United States frequently assails the rights record of rival China but has comparatively warm relations with Vietnam, which despite war memories has stepped up defense cooperation with Washington in the face of a rising Beijing.

Vietnam has detained a number of high-profile journalists and activists in recent months ahead of the regime's five-yearly party congress, slated for next January.

sct/jm