(CNN) Vietnam has jailed 12 members of a US-based anti-government organization with roots in the Vietnam War, including two US citizens accused of plotting to bomb an international airport.

Those jailed included Nguyen James Han and Phan Angel, both US citizens. They will be deported from Vietnam after the completion of their 14 year jail terms, the Vietnam News Agency said.

Pope Thrower, a US embassy spokesman in Hanoi, said consular staff will "continue to monitor Mr. Nguyen's welfare, advocate for him, and provide consular services until his release."

"We have no higher priority than the safety and security of US citizens abroad," Thrower added. The embassy has not been authorized to speak on behalf of Phan.

While Vietnam routinely jails bloggers, activists and rights lawyers who are critical of the government, Nguyen and Phan's case has shone a light on the continued operation of US-based opposition groups, decades after the end of the Vietnam War and normalization of relations between Hanoi and Washington.

At least two of those groups have been accused of carrying out terrorist activities in Vietnam with the purpose of achieving regime change in the communist nation.

However, details about the groups and the severity of their actions remain unclear, with both the Vietnamese government and the organizations themselves having a vested interest in exaggerating their role and influence.

Nguyen James Han, an American citizen who is a member of California-based Provisional National Government of Vietnam was sentenced to 14 years in prison.

Aspiring government in exile

Both Nguyen and Phan belong to the Provisional National Government of Vietnam (PNGV). The organization was formed in the early 90s by former soldiers and officials from the former US-sponsored South Vietnam, which was defeated by the North in 1975.

The PNGV is headed by self-proclaimed Prime Minister Dao Minh Quan and not recognized as an official government in exile by any other countries.

Vietnamese state media claims that after the group's establishment, Quan and other leaders recruited followers in Vietnamese refugee camps in Hong Kong and other parts of Southeast Asia. "The recruited refugees were later repatriated in order to conduct terror and sabotage activities in Vietnam," according to the Vietnam News Agency.

PNGV's website says it grew out of the New Democracy Movement, which formed in 1985 to demand "free and fair election(s) for Vietnam."

"One of the important goals of the New Vietnamese Democracy is to unify all the hope and desire of the freedom-loving Vietnamese to become a National Resolution to discharge the dictatorship of communist in Vietnam," it says.

The website prominently features letters from various US Presidents and other high-ranking officials which refer to Dao as Prime Minister, most of which appear to be pro forma replies to letters sent by members of the PNGV.

For example, a 2005 letter signed by Melissa Bennett, special assistant to then President George W. Bush, apologizes for a delay in responding to an unspecified invitation from the PNGV, and communicates Bush's "best wishes."

Quan did not immediately respond to a request for comment about the imprisonment of Nguyen and Phan or information about his group's activity in Vietnam.

Alleged terrorists

According to Vietnamese authorities , the PNGV has been active within Vietnam itself since 2015, when the group's leaders in the US "instructed underground units in Vietnam to develop forces and establish 'volunteer delegations,' seek venues to set up bases, and purchase weapons to prepare for spontaneous and terror acts."

Vietnam's Ministry of Public Security claims that last year these efforts expanded to "aiming to terrorize, sabotage and assassinate officials," as well as carrying out attacks on police agencies and an attempted bombing of Ho Chi Minh City's Tan Son Nhat airport.

The court heard this week that Nguyen and Phan -- the US citizens convicted -- were sent to the country "to develop their force and direct other members in the country to conduct anti-state activities."

These initially involved small scale events such as "distributing leaflets, hacking into radio stations to broadcast information about their organisation, and protesting against Taiwanese steel maker Formosa," but later escalated to "armed terrorist plots."

In sentencing the 12 PNGV members, the court said they had set fire to police stations, planted petrol bombs at Ho Chi Minh City's Tan Son Nhat airport, and attacked a police station, injuring three people.

Last year, 15 people were jailed for their part in the airport bombing plot, according to Reuters , citing state media reports.

Anti-communist activity

While much of the PNGV's history is unclear, beyond the group's own accounts and limited information provided by the Vietnamese government, it seems to have filled something of a vacuum left by the self-proclaimed Government of Free Vietnam (GFV), which was founded in 1995 but has since disbanded.

An archived version of GFV's website called for dismantling "the suppressing system of Communist dictatorship of the Socialist Republic of Vietnam by a peaceful, democratic, humanity, practical and persistent approach."

Founded by former members of the South Vietnamese government and armed forces, the GFV, like the PNGV, claimed to be the natural successor to that administration, arguing the Socialist Republic of Vietnam formed after the end of the war is illegitimate.

A 2003 report from the US Justice Department describes the GFV as a "dissident group based in California," which reportedly operates "armed groups in bases within Laos and Cambodia along the Vietnam border."

"The group has claimed responsibility for a number of attempted bombings throughout Vietnam," the report said, adding the "Californian basis of the group has led to formal complaints to the US government by the Vietnamese government."

While the US has a long history of funding dissident and armed groups both overseas and at home, leaked State Department cables published by WikiLeaks suggest the GFV was not held in high regard by diplomats in Hanoi.

"On the specific issue of the (GFV's) office in Washington, the Ambassador remarked that opening an office is not illegal and, as far as we could tell, would have virtually no real significance in terms of damaging (Vietnamese) interests," a 2005 cable said

A later cable noted there was evidence GFV members "have plotted and carried out terrorist attacks on Vietnamese diplomatic missions in England, Thailand, Cambodia and the Philippines."

An immigration lawsuit in 2014 involving a member of GFV gives some insight into how those activities were carried out. In a judgment in favor of Vinh Tan Nguyen, the court recounted how he had served as a "vice commander" of a GFV jungle camp along the Cambodian-Vietnamese border.

Nguyen's brother tried to bomb the Vietnamese Embassy in Bangkok in early 2001, and he himself was arrested in the Philippines months later while in the planning stages of an attack against the Manila embassy.

While GFV has a long paper trail in leaked state department cables, government reports and lawsuits, PNGV is not mentioned in the WikiLeaks documents, nor was CNN able to find any reference to the group on US government websites.