A US citizen was sentenced Monday to 12 years in Vietnamese prison for "attempting to overthrow the state", a lawyer and state media said, as the one-party country squeezes dissent.

The trio were accused of setting up a group to prepare armed protest and the occupation of official "headquarters" in Hanoi and Ho Chi Minh City, according to state-run news site VnExpress.

Nguyen was convicted during a half-day trial in a Ho Chi Minh City court, given 12 years and ordered to leave the country after completing his sentence, a lawyer for one of the group said.

The two activists arrested with him, Tran Long Phi and Huynh Duc Thanh Binh, were jailed for eight years and ten years respectively for the same charge.

"The sentence for the defendants is too harsh," lawyer Nguyen Van Mieng said.

State media also reported that police are looking for another Vietnamese man who had connections with Michael Nguyen and is currently on the run.

After leaving Vietnam in 1975, Michael Nguyen settled in the US alongside more than a million people who fled when the communist regime took over after the Vietnam War.

Many war refugees mobilised against the communist government from afar, but Nguyen's family have said he is not involved with any dissident groups.

Vietnam has routinely jailed its critics since a new administration came to office in 2016, with nearly 130 prisoners of conscience presently behind bars in the country.

A cybersecurity law passed last year gives authorities unprecedented powers to police online content and has come under fire from the US, the EU and the UN.

The bill requires internet companies like Facebook and Google to hand over user data and remove material from their sites when requested by the government.

(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)