HOUSTON — If the Trump administration gets its way, U.S. citizens boarding international flights will have to submit to a face scan, a plan privacy advocates call a step toward a surveillance state.

The Department of Homeland Security says it’s the only way to successfully expand a program that tracks nonimmigrant foreigners. They have been required by law since 2004 to submit to biometric identity scans — but to date have only had their fingerprints and photos collected prior to entry.

Now, DHS says it’s finally ready to implement face scans on departure — aimed mainly at better tracking visa overstays but also at tightening security. But, the agency says, U.S. citizens must also be scanned for the program to work.

Privacy advocates say that oversteps Congress’ mandate.

“Congress authorized scans of foreign nationals. DHS heard that and decided to scan everyone. That’s not how a democracy is supposed to work,” said Alvaro Bedoya, executive director of the Center on Privacy and Technology at Georgetown University.

Trials are underway at six U.S. airports — Boston, Chicago, Houston, Atlanta, Kennedy Airport in New York City and Dulles in the Washington, D.C., area. DHS aims to have high-volume U.S. international airports engaged beginning next year.

During the trials, passengers will be able to opt out. But a DHS assessment of the privacy impact indicates that won’t always be the case.

“The only way for an individual to ensure he or she is not subject to collection of biometric information when traveling internationally is to refrain from traveling,” says the June 12 document on the website of Customs and Border Protection, which runs the DHS program.