The restrictions begin at 11:59 p.m. Friday, Eastern time.

By midnight Friday, the new restrictions will be in full force.

But they will not apply to anyone aboard a flight to the United States that left before that time.

Hours after the announcement, it was still unclear how flights that operate regularly between Europe and the United States would be affected, and airlines were scrambling to create contingency plans.

American Airlines said that customers holding U.S. passports or Green Cards attempting to re-enter the United States from Europe would be rebooked, if necessary, to one of the approved airports. But the carrier added that any other passengers who had traveled to one of the Schengen countries within the past two weeks would not be allowed to travel to the United States on its flights.

Air France said it would announce on Thursday whether it would adjust flights as a result of the restrictions. But Norwegian Airlines, a low-cost carrier with several routes between Europe and the United States, said its flights would continue as normal in the short term.

Not all foreign travelers will be subjected to the travel restrictions. The spouses, parents or siblings of American citizens or permanent residents, will be exempt, as will members of the U.S. military and their spouses and children.