Among the nine accused of leading the plot are a former Laotian general and a Lieutenant-Colonel Harrison Ulrich Jack, a retired California National Guard officer who reportedly served in covert operations during the Vietnam War.

Aside from Jack, the eight suspects are all prominent members of the ethnic Hmong community who have settled in prefix="st1" ns="urn:schemas-microsoft-com:office:smarttags"? California 's Central Valley after fleeing South-East Asia.

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Prosecutors said several key buildings in

Vientiane would have been attacked [EPA] All nine suspects face life imprisonment if convicted.

Speaking in court, Twiss said that while those charged were believed to be the main leaders of the plot, thousands of co-conspirators remain at large, many in other countries.

He said that as recently as May this year, agents acting on behalf of the plotters had been gathering intelligence about military installations and government buildings in the Laotian capital, Vientiane.

A statement from the California public prosecutor's office said the Hmong planned to use a range of rifles, Stinger surface-to-air missiles, anti-tank rockets and other arms and munitions to "reduce government buildings in Vientiane to rubble".

'Tarnished Eagle'

The arrests followed a six-month investigation by police and anti-terrorism authorities dubbed "Operation Tarnished Eagle".

According to police, the nine, most aged in their fifties and sixties, were heard during covert surveillance discussing plans to buy hundreds of weapons and ship them to Laos via Thailand .

Thousands of Hmong refugees fled Laos

at the end of the war [GALLO/GETTY] The plotters were also said to have met an undercover agent posing as an arms broker during which they agreed to pay $150,000 for a consignment of weapons.

"Fortunately, we were able to disrupt their activities before their plot evolved into a coup against a country with which the United States is at peace," said Michael Sullivan, the federal official who headed the probe.

"These defendants posed a substantial threat to public safety abroad."

Among the nine charged is Vang Pao, a prominent figure in the Hmong community in the United States .

A former general in the Royal Lao army in the 1960s and 1970s and an ethnic Hmong, he fled to the US in 1975 after communist forces ousted the pro-American government in Laos .

Prosecutors say Vang Pao was the likely ringleader of the plot.

Rights groups have accused Laotian authorities of persecuting the Hmong hill tribe groups because of their association with resistance fighters opposed to the communist government.