By Bill Hayton
BBC News, Hanoi
Vietnam prosecutors say they intend to charge three US citizens and four others with terrorism, accusing them of trying to overthrow the government.
At least two have been detained for over a year, including Nguyen Thuong Cuc from the state of Florida.
A Florida senator has threatened to block a law normalising trade ties with Vietnam until the case is resolved.
The issue may complicate a visit by US President Bush to Vietnam set for two weeks time.
The US embassy in Hanoi has confirmed that the three US citizens - Nguyen Thuong Cuc, alias Cuc Foshee; Huynh Bich Lien, alias Linda Linh; and Le Van Binh - may be facing terrorism charges.
It is checking the identity of a fourth person, Cao Tri.
Local media reports that all of them are Vietnamese Americans, residents of the states of California and Florida.
'Inciting an uprising'
It is believed they may be involved with a group called the Government of Free Vietnam, which the communist authorities consider a terrorist organisation.
As a result of the case, a Florida senator has threatened to block the bill normalising trade relations between the US and Vietnam, which the Bush administration wants passed before the president visits Hanoi in two weeks time.
The bill's failure would be embarrassing for both governments, which have heralded it as symbolising their new partnership.
The Vietnamese authorities say they have also arrested three other people in connection with the case - all of them Vietnamese citizens from the southern province of An Giang.
Prosecutors say all seven were plotting to take over radio stations and incite an uprising against the Communist Party.
They said the leader of the alleged plot was Nguyen Huu Chanh, who they recently tried and failed to have extradited from South Korea.
The Government of Free Vietnam advocates the use of armed force and is said to have a military base somewhere on the Vietnam-Cambodia border.
However there is little evidence that it has any significant support inside Vietnam.