Major airlines say that most passengers whose flights are canceled, and many who choose not to fly, will be eligible for refunds and no-fee rebookings.

A small percentage of domestic flights were canceled on Friday, but Transportation Secretary Sean Duffy said the reductions, which were already going to increase next week, could sharply rise by the holidays if the shutdown continued.

Major airlines are complying with the FAA's call to reduce flight schedules starting Friday, with some carriers already beginning to cancel flights.

Thousands of flights will be cancelled at forty airports across the country, including Denver International Airport.

The rail service said it began to see those bookings grow before the shutdown began — but told The Associated Press on Thursday that it was also “reasonable to expect an increased share shift to Amtrak” if flight disruptions mount. Amtrak has maintained that its routes are running normally amid the shutdown.

The FAA will reduce air traffic at 40 airports beginning Friday. Philadelphia International Airport is among those expecting flight cuts and delays. Talks between Democrats and Republicans to end the shutdown are intensifying,

Hundreds of flights have been canceled as FAA reductions take effect at high-traffic airports, including Philadelphia's PHL.