HANOI (Reuters) - Vietnam has declared a U.S.-based activist group a terrorist organization and warned that any Vietnamese found to be involved with the group would be regarded as co-conspirators and punished.

The government said the California-based Viet Tan, or Vietnam Reform Party, had recruited and trained operatives to use weapons and explosives.

Vietnam has long been sensitive to the activities of Viet Tan, calling the group “reactionaries” but the announcement carried on state television was the first time it had designated it a terrorist organization.

Vietnam’s police-run Ministry of Public Security said Viet Tan had trained members in militant activities, kidnaps and murders and arranged for operatives to sneak in to Vietnam to organize protests and instigate violence.

The group in a statement said the government feared organized opposition and the police were “regurgitating baseless propaganda” to try to deter Vietnamese from “peaceful political advocacy.”

“Let the people of Vietnam decide whether Viet Tan is a threat,” it said.

The U.S. State Department said Viet Tan was not listed as a terrorist entity under U.S. law. “We would refer you to the Vietnamese government for more information on its designation,” said Katina Adams, a spokeswoman for the department’s East Asia bureau.

Viet Tan has long been an annoyance for the Communist Party that has ruled since the U.S.-backed South Vietnam government fell to northern forces in 1975, leading to an exodus of more than 1 million people, mostly to the United States.

It was founded by exiled remnants of the deposed Saigon government in 1982 and states as its mission to “overcome dictatorship and build the foundation for a sustainable democracy”.

Despite steadily introducing more liberal social and economic reforms in recent years, the Communist Party has a zero-tolerance approach to criticism and has punished detractors harshly.

Experts said the huge Vietnamese appetite for social media creates a challenge for the authorities to keep a lid on dissent, as seen this year during widespread protests over an environmental disaster in which demonstrators were mobilized largely via Facebook.