This week, U.S. Customs and Border Protection unveiled new rules that bar foreign citizens from traveling to the U.S. under the Visa Waiver Program (VWP) if they have visited North Korea after March 1, 2011.

The new rules would apply to citizens of 38 countries where the VWP currently applies, including South Korea. Travelers from those countries would still be eligible to apply for a visa to visit the U.S., and would not be barred from visiting outright.

The new rules appear to have been triggered by the U.S. re-listing of North Korea as a State Sponsor of Terrorism in November 2017. (North Korea was previously on this list from 1988 through 2008.) The “Visa Waiver Program Improvement and Terrorist Travel Prevention Act of 2015,” which was passed into law in December 2015 as a rider to the 2016 Consolidated Appropriations Act, bars visa-free travel to the U.S. by foreign nationals who have visited Iraq, Syria, any countries designated as a State Sponsor of Terrorism, or other countries of concern.

It is unclear, however, why there was a twenty-month delay between North Korea’s State Sponsor of Terrorism designation and the adoption of these new rules.

Under the law, ineligibility for the VWP may be waived for foreign nationals who traveled to a listed country for purposes related to military or government service, or if the Department of Homeland Security determines that such a waiver “is in the law enforcement or national security interests of the United States.” In 2016, DHS announced certain criteria under which those waivers might apply:

As a general matter, categories of travelers who may be eligible for a waiver include individuals who traveled to these countries on behalf of international organizations, regional organizations, and sub-national governments on official duty; on behalf of a humanitarian NGO on official duty; or as a journalist for reporting purposes.

At this time, it is not certain whether such humanitarian or journalistic waivers would be applied to individuals who had traveled to North Korea for these purposes.

Yonhap News, citing sources from the South Korean Foreign Ministry, estimates that about 37,000 South Koreans have traveled to North Korea since March 2011 and would therefore now be required to apply for a visa for any travel to the United States. As NK News reports, the new rules could also deter non-U.S. tourism to North Korea.