The Pentagon on Friday ordered more security screening for legal residents seeking a fast track to U.S. citizenship in exchange for military service.

The policy change comes after concerns were raised to Defense Secretary Jim Mattis in July that the Military Accessions Vital to the National Interest program, or MAVNI, could pose a security risk. About 1,800 foreign-born recruits signed up to gain citizenship under the program.

Certificates for expedited citizenship that had been granted to troops will be revoked until they complete the stepped-up screening requirements, the department said.

"While the department recognizes the value of expedited U.S. citizenship achieved through military service, it is in the national interest to ensure all current and prospective service members complete security and suitability screening prior to naturalization," according to a DOD release.

Lawful permanent residents who are part of the MAVNI program must now pass a background screening before starting their military service, the department said. The recruits had been allowed to start service while the check was ongoing.

They will also be required to complete 180 days of military service — instead of just one day — before being given a certificate for expedited citizenship.

The MAVNI program recruits foreign residents who are in the U.S. with a legal immigration status or fall under the Deferred Action for Childhood Arrivals policy. It was designed to enlist troops with unique and valuable skills such as language fluency.

Democrats on Capitol Hill feared the program would be cancelled and participants with expired visas would be forced to return to their native countries. Sen. Mark Warner, D-Va., had threatened "strong, swift action" if enlistment contracts were cancelled.