Ads appearing last month in two of the largest Vietnamese-language newspapers in the country, and picked up by radio and television, sought help against the "proliferation of Vietnamese Communist intelligence". The FBI is now following up hundreds of calls received in response.
The US and Vietnam opened full diplomatic relations last year, but the agency says there has lately been a surge in espionage. In a curious throwback to the Vietnam War era, the newspaper notices asked new immigrants to help the US government "destroy the activities, threat, and intimidation . . . of the underground Communist spies." Economic espionage is the main target.
The Vietnamese-American community, estimated at about 1 million, has seen sharp splits between ardent anti-Communists who denounce Hanoi and those who welcome closer ties with their home country.
The San Francisco FBI spokesman, George Groltz, said the campaign was intended to encourage disgruntled Vietnamese spies to "come in from the cold". Mr Groltz said known Vietnamese spies had tried to obtain security clearances and access to hi-tech companies based in California.