The Department of Homeland Security is adding new requirements for countries that participate in the Visa Waiver Program as part of the Trump administration's ongoing efforts to tighten border and travel security.

DHS announced Friday that the 38 countries participating in the program will now be required to use U.S. counterterrorism information to screen travelers crossing their borders, as part of their existing information-sharing agreements.

The U.S. will also begin assessing how effectively the countries are protecting against insider threats to aviation security by making sure they vet their airport employees and ensure those employees aren't corrupted.

They'll also require Hungary, Greece, Portugal and San Marino — four countries whose citizens stayed in the U.S. longer than permitted at a rate of two percent or more last year — to launch public information campaigns to educate their citizens on the specifics of the program and consequences of violating its conditions.

The Visa Waiver Program allows citizens of 38 countries to travel to the U.S. for business or tourism for up to 90 days without having to obtain visas. About 20 million people travel on the program every year.

"The United States faces an adaptive and agile enemy, as terrorists continue to explore ways to reach our country and to direct, enable, and inspire attacks against us," the agency's new secretary, Kirstjen Nielsen, said in a statement. "It's critically important we stay ahead of these threats by improving our security posture."

As part of the roll-out, DHS is also calling on Congress to pass legislation to make permanent a series of measures already in place, such as requiring countries to allow U.S. Federal Air Marshals to operate on planes headed to the U.S.

Senior administration officials stressed they expect the countries to comply voluntarily. But if they don't, the U.S. could take a number of steps, including, at last resort, ending their membership in the program.

Officials said a number of countries are already in compliance, but declined to say how many or which ones.