Rights groups say at least 15 dissidents have been arrested since January in Vietnam, where a new administration in charge since last year has been accused of tightening already tough controls on freedom of expression.

Catholic activist Nguyen Van Oai was sentenced to five years in jail on Monday for failing to comply with terms of his house arrest and resisting public officers, his lawyer told AFP after the half-day trial in the central province of Nghe An.

"The court delivered a very unfair verdict based on its biased views," attorney Ha Huy Son said, adding that Oai would appeal the verdict.

Oai, 36, was convicted in 2013 along with 12 other activists of plotting to overthrow the government -- a common charge levied against government critics.

He was sentenced to four years in jail and ordered to serve another four years under house arrest.

He was re-arrested in January after he violated the terms of his house arrest and "strongly resisted" the orders of local officials, the state-run Ho Chi Minh City Law online newspaper said.

In a letter written ahead of the trial, Oai's wife maintained her husband's innocence and said he stood for "freedom and human rights".

"He is innocent. He is only guilty of refusing to remain silent in the face of misconduct (by authorities)," Ho Thi Chau wrote in the letter posted on Facebook last month.

Vietnam has long been criticised by advocacy groups for its dismal human rights record. It routinely jails bloggers, lawyers and activists accused of criticising the state.

Though all media in Vietnam is government-controlled, activists have turned to social media in recent years to voice grievances and promote freedom of expression.

Many have been arrested or handed heavy jail terms as a result.

In June prominent blogger Nguyen Ngoc Nhu Quynh, known as "Mother Mushroom", was sentenced to 10 years in jail for anti-state propaganda for her online posts about the environment and politics, including an investigation into deaths in police custody.

The following month anti-China activist Tran Thi Nga was given nine years in jail for posting online articles deemed critical of the state.

In late July four dissidents were arrested on charges of trying to overthrow the government. The group was connected to a lawyer and activist who were previously arrested on the same charge, which carries the death penalty.

Amnesty International says at least 15 activists and government critics have been detained since January this year.

Agence France Presse - September 18, 2017