HANOI (AFP) - Vietnam declared a US-based group a "terrorist organisation" on Tuesday (Jan 30), a rare designation in the communist country, after some members were convicted for plotting to attack Ho Chi Minh City's airport.

Though Vietnam heavily cracks down on critics, few anti-government organisations or activists have been jailed for terrorism, which is punishable by death in the authoritarian state.

Vietnam's police-run security ministry on Tuesday labelled the California-based Provisional National Government of Vietnam (PNGV) terrorists, just weeks after 15 of its members were convicted of planting petrol bombs in 2017 at Tan Son Nhat international airport, the country's busiest.

"The PNGV is a terrorist organisation," the Ministry of Public Security (MPS) said on its website, adding that anyone found promoting or colluding with the group would be "punished in accordance with Vietnamese law".

PNGV emerged in Vietnam from relative obscurity following the December 2017 trial uncovering the foiled airport attack, which the judge said had been funded from abroad.

The group, formed in 1991, says its stands for free and fair elections in Vietnam, which it labels a "dictatorship" on its website.

Some of its members include former officials from the US-backed southern regime who fled to the United States after the end of the Vietnam War, according to MPS.

Five PNGV members were arrested in 2015 Vietnam for allegedly trying to buy weapons to commit terrorist acts, it added.

PNGV did not immediately respond to a request for comment.

Scores of dissidents are behind bars in Vietnam, many serving sentences for "anti-state propaganda" or attempting to overthrow the government, though terrorism convictions are not common.

Most are in jail for speaking out against the communist government or pushing for peaceful reform, not political violence.

In 2016, Vietnam labelled the US-based Viet Tan anti-government group a "terrorist organisation" for "instigating violence", a charge the group rejects.