US makes exceptions for North Korea travel ban

— -- The State Department has announced it will allow four categories of Americans to apply for an exemption after it bars most citizens from traveling to North Korea starting next month.

The ban will go into effect on Sept. 1 and will make it a felony to use a U.S. passport to travel to North Korea.

The ban comes after American student Otto Warmbier was arrested and imprisoned for 17 months in North Korea, during which time he went into a coma. Wambier died on June 19, shortly after being returned to the U.S.

The four categories of people exempt from the ban are journalists traveling to cover North Korea, American Red Cross or International Committee of the Red Cross employees on official business, other aid workers with "compelling humanitarian considerations" and anyone else whose trip "is otherwise in the national interest."

Waivers will be valid for one round trip to the country. The State Department estimates 100 people will apply for the waivers.

The department posted its official notice of the policy in the Federal Register today, which means that there are now 21 days for the public to comment on it and 30 days until it may be implemented.

Secretary of State Rex Tillerson announced the ban on July 21 amid outrage and heightened concerns about Americans' safety in North Korea after Warmbier's death.

Warmbier was a 21-year-old University of Virginia student who was arrested in North Korea in January 2016 while visiting the country as part of a tour group. At some point during his imprisonment, he went into a coma, but the cause is still unclear. He was evacuated by American officials but died days after being returned to the U.S.

North Korea is known to be holding at least three other Americans, two of whom were working at Pyongyang University of Science and Technology when they were detained this spring.

There are a few dozen U.S. citizens who work at the institution, which is the country's only private university and whose staff includes 60 to 80 foreigners throughout the academic year.

It's unclear if those Americans will be permitted to continue working in North Korea. Some are dual nationals who could travel using their other passports. The State Department specified in its notice that "use of a U.S. passport" will be invalid.

Each year, 800 to 1,250 Americans visit North Korea, although that number is expected to decline sharply this year, after Warmbier's death and the travel ban.

The ban is officially known as a geographic travel restriction, which gives the secretary of state the ability to bar travel to an area or a country if "the country is at war with the United States, armed hostilities are in progress in the country or area or there is imminent danger to the public health or physical safety of U.S. travelers in the country or area," according to the notice filed by the State Department.

ABC News' Ben Gittleson contributed to this report.