United States officials on Thursday announced new restrictions regarding the country’s Visa Waiver Program, an action designed to prevent foreign terrorists from entering the nation.

The Department of Homeland Security said it added Libya, Somalia and Yemen to its list of “countries of concern,” which means most people who have traveled to those nations since March 2011 will not be allowed to enter the United States through the Visa Waiver Program.

That program allows citizens and nationals of 38 countries to visit the United States without first securing a visa, so long as they stay for fewer than 90 days. But lawmakers sought to reform the program after the Nov. 13 Paris terrorist attacks, and the changes went into effect last month when officials announced similar travel restrictions concerning Iran, Iraq, Syria and Sudan.

The White House has said the 20 million people who enter the United States through the program every year were already being screened to prevent terrorists and other potential security threats from entering the country. But those who sought to reform the program spoke about its potential vulnerabilities, and the new rules add an extra layer to the vetting process.

In making Thursday’s announcement, the Department of Homeland Security said “many” foreign terrorists are nationals of the countries that participate in the Visa Waiver Program.

The State Department has issued active travel warnings for all seven aforementioned countries, in many cases citing terrorist or other extremist activities.

The new restrictions do not ban people who have traveled to the seven countries since March 2011 from entering the United States altogether. However, they are now required to first apply for a visa at a United States embassy. That process includes an in-person interview.

Homeland Security Secretary Jeh Johnson can grant waivers on a case-by-case basis, and the department said such waivers may be available for people like journalists or humanitarian aid workers.