Michael Nguyen, who was born in Vietnam, has lived in the US since he was a child (Reuters)

A court in Vietnam on Monday sentenced a U.S. citizen to 12 years in prison after finding him guilty of "attempting to overthrow the state" in a trial that lasted just half a day, his lawyer said.

Michael Phuong Minh Nguyen, 55, pleaded guilty, but had asked that his sentence be reduced so he could be reunited with his family, his lawyer Nguyen Van Mieng told Reuters by telephone after the trial at the Ho Chi Minh City court.

Nguyen will be deported after serving his jail sentence.

"It's such a long sentence," said Mieng. "Michael admitted guilt at the trial and asked the jury to reduce his sentence so that he could soon reunite with his family."

Nguyen was accused of inciting Vietnamese people to join protests and attempting to attack government offices in Hanoi and Ho Chi Minh City with Molotov cocktails and slingshots, the Tuoi Tre newspaper said, citing the indictment.

"We are disappointed by today's verdict," a spokeswoman for the U.S. Embassy in Hanoi said in an emailed statement. "We will continue to raise our concerns regarding Mr. Nguyen's case, and his welfare, at all appropriate levels."

Nguyen, who was born in Vietnam and has lived in the United States since he was a child, was detained in Vietnam in July 2018 on suspicion of activity against the government, said his brother-in-law, Mark Roberts.

Despite sweeping economic reform and increasing openness to social change, Vietnam's ruling Communist Party retains tight media censorship and does not tolerate criticism.

The court also sentenced two Vietnamese men to eight years and 10 years in prison for the same offence, Mieng said, adding that they will face three years' house arrest after their jail terms.

"Their objective is to cause riots in an attempt to overthrow the administration of Vietnam and eradicate the leading role of the (Communist) party," Truoi Tre cited the indictment as saying.