WASHINGTON — A U.S. ban on Americans traveling to North Korea took effect Friday amid concerns about the fate of those who have been detained there in the past.

The U.S. said its citizens can start applying for exceptions, but few will be granted.

The ban, announced by Secretary of State Rex Tillerson in July after the death of American student Otto Warmbier following his release from North Korea, makes U.S. passports invalid for travel to the North.

Americans who have a valid reason to travel to the North can still go under “extremely limited” circumstances, the State Department said, adding that applicants must prove that their trip is in the U.S. national interest. Professional journalists assigned to collect information for public consumption about North Korea might be eligible, along with Red Cross representatives on officially sponsored missions. Humanitarian workers also could receive exemptions.

In new details released Friday about the exemption process, the State Department said applicants must email or mail a statement explaining why their trip serves the national interest, along with documentation to substantiate it. Applicants must also send a copy of their identification and contact information.

The State Department will notify applicants whether they’ve been deemed eligible or not.