A plane lands at Newark International Airport in Newark, N.J., Thursday. (AP Photo/Seth Wenig)

U.S. airlines began canceling hundreds of flights Thursday due to the Federal Aviation Administration’s order to reduce traffic at the country’s busiest airports starting Friday because of the government shutdown.

More than 500 flights scheduled for today were already cut nationwide, and the number of cancellations climbed steadily throughout Thursday, according to FlightAware, a website that tracks flight disruptions.

The FAA order to cut flights at 40 of the busiest airports across the U.S. includes New York, Los Angeles and Chicago, according to a list distributed to the airlines and obtained by The Associated Press. But the impact will disrupt travel at many smaller airports too.

The FAA seeks to reduce service by 10% across “high-volume” markets to maintain travel safety as air traffic controllers exhibit signs of strain during the shutdown. The move also comes as the Trump administration is ramping up pressure on Democrats in Congress to end the shutdown.

Airlines will phase in the 10% reduction in their flight schedules at the airports across more than two dozen states. Just hours before the reductions went into place, airlines were scrambling to figure out where to cut and some travelers began changing or canceling itineraries preemptively.

Travelers with plans for the weekend and beyond waited nervously to see if their flights would take off as scheduled.

The affected airports included busy connecting hubs and those in popular tourist destinations, such as Atlanta, Denver, Orlando, Miami and San Francisco. In some of the biggest cities — such as Dallas, Houston and Chicago — multiple airports will be affected.

Airlines will phase in the cuts at the direction of the FAA, eliminating 4% of flights at the 40 airports today and working up to 10%, according to three people familiar with the plans who were not authorized to discuss them publicly.

Delta Air Lines said it planned to cancel roughly 170 flights today. By late Thursday afternoon, United Airlines had called off 145 flights and American Airlines had canceled 32 for Friday, according to aviation analytics firm Cirium.

The FAA had not yet published an official order as of the afternoon.

Some airlines planned to focus on slashing routes to and from small and medium-size cities.

“This is going to have a noticeable impact across the U.S. air transportation system,” industry analyst Henry Harteveldt said.

The flight reductions just weeks before the busy holiday season prompted some travelers to change their plans or looking at other options.