WASHINGTON (AP) — Foreigners in the U.S. who want a green card will need to leave and apply in their home country, the Trump administration announced Friday, in a surprise change to a longstanding policy that sowed confusion and concern among aid groups, immigration lawyers and immigrants.

For over half a century, foreign nationals with legal status have been able to apply for and complete the entire process for permanent residence in the United States — including individuals married to U.S. citizens, holders of work and student visas, and refugees and political asylum seekers, among others.

READ MORE: What protections do green card holders and foreign students have in the U.S.?

The announcement from U.S. Citizenship and Immigration Services is the latest step by the Trump administration making legal immigration more difficult for foreigners already in the U.S. and for those hoping to come here.

The changes comes on top of steps the administration has already taken to restrict and limit entry for people from dozens of countries. In some cases, there are outright bans on travel from those countries, while people from others face pauses in visa processing. Experts and attorneys warned that forcing people from those countries to return home to apply for a green card would result in them being barred from coming back.

"If families are told that the non-citizen family member must return to his or her country of origin to process their immigrant visa, but immigrant visas are not being processed there, it's a Catch-22. These policies will effectively create an indefinite separation of families," wrote World Relief, a humanitarian and refugee resettlement organization.

In the announcement, USCIS said foreigners who are in the U.S. temporarily and who want to apply to become lawful permanent residents, or green card holders, have to return home and apply there, except in "extraordinary circumstances," without elaborating. USCIS officers would decide whether applicants meet those.

"Nonimmigrants, like students, temporary workers, or people on tourist visas, come to the U.S. for a short time and for a specific purpose. Our system is designed for them to leave when their visit is over. Their visit should not function as the first step in the Green Card process," the agency said in a statement.

The USCIS announcement did not say whether individuals would be required to remain in another country throughout the entire process, or whether the policy impacts foreigners whose green card applications are already underway.

The agency did not respond to emailed questions about how many people would be affected or criticism of the news.