WASHINGTON — A U.S. prohibition on Americans traveling to North Korea has taken effect.

The ban makes U.S. passports invalid for travel to North Korea and allows the State Department to revoke the passport of anyone who flouts the ban. It was announced in July and kicked in Friday.

Only those granted “extremely limited” exceptions are exempt. The State Department says applicants must prove their trip is in the U.S. national interest. That could include journalists and aid or humanitarian workers.

The State Department says applicants must submit a statement explaining why the trip serves the national interest, along with documentation.