HANOI (Reuters) - Vietnam has declared a United States-based activist group a terrorist organisation 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 organisation.

The 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 organise protests and instigate violence.

Viet Tan has long been an annoyance for the Communist Party that has ruled since the US-backed South Vietnam government fell to northern forces in 1975, leading to an exodus of more than one 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".

Viet Tan did not immediately respond to a request for comment.

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 say 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 mobilised largely via Facebook.