The White House announced changes Monday to a program waiving visas for some tourists after this month's deadly attacks in Paris raised fresh concerns about foreign radicals coming to the United States.

The U.S. is “aggressively strengthening” the tourism program and “bolstering our relationships” with the 38 countries that participate, the White House said in a fact sheet on Monday.

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The program allows roughly 20 million people a year to visit the country from nations such as Japan, Australia and Germany.

Those tourists are screened against American databases, but are subjected to much less stringent checks than other travelers. That has sparked concerns that radicalized militants could try and take advantage of the situation to launch attacks in the U.S.

Among other steps, the Department of Homeland Security (DHS) will seek to determine whether any travelers to the U.S. had visited “countries constituting a terrorist safe haven,” the White House said.

DHS, the FBI and other arms of government will also offer support to countries participating in the program so they can more thoroughly screen refugees and other migrants. The U.S. will also provide “surge teams” to help those countries block radicals from crossing their borders.

Additionally, federal agencies are being directed to look for ways to beef up the program’s screening of fingerprints, photographs and other biometric data, as well as reviewing how well the U.S. and other countries share intelligence.

The administration continuously updates the program “to address current threats,” the White House said on Monday, including a new requirement this summer that tourists use an e-Passport under the program.

“In the wake of the attacks in Paris, the administration is announcing additional actions today that will further enhance and accelerate these changes,” the White House said on Monday.

The string of terrorist attacks on Nov. 13 that killed 130 people across Paris prompted new focus on the possible threat posed by refugees from the civil war in Syria, following indications that at least one of the attackers disguised himself as a refugee in order to enter Europe.

However, many national security officials say that a much bigger concern is the Visa Waiver Program, given the already extensive vetting process for refugees and the fact that all of the Paris attackers who have been identified were citizens of France or Belgium.

Efforts to tighten the program are likely to meet fierce resistance from the tourism industry, however, which relies on the money spent by millions of foreign visitors each year.

The White House’s move on Monday comes amid talk of new legislation from Congress.

The Obama administration has said it would support those plans, and on Monday laid out a series of steps that Congress could take.

It suggested that Congress act to increase fines for airlines that don’t verify a traveler’s passport information and also work to boost the sharing of information with foreign agencies.