Full List: 40 Airports Facing Flight Cuts

The Federal Aviation Administration will reduce flight capacity by 10% at 40 major airports across the country, officials announced during a press conference on Wednesday.

The decision could cut thousands of flights per day.

The restrictions will go into effect Friday morning, FAA Administrator Bryan Bedford and Transportation Secretary Sean Duffy said.

ABC News obtained the list of airports, which includes Atlanta's Hartsfield-Jackson, Boston Logan, all three New York City-area airports, Chicago O'Hare and many others. Sources said the cuts could start as early as Thursday.

A source told ABC News that the flight reductions will start at 4% on Friday and work up to 10%. The flights impacted by these reductions are scheduled during the hours of 6 a.m. to 10 p.m.

International flights are exempt from the cuts, the source said, adding that a final list of airports and the FAA order with details is still in the works and is expected to be issued Thursday.

"Our sole role is to make sure that we keep this airspace as safe as possible. Reduction in capacity at 40 of our locations. This is not based on light airline travel locations. This is about where the pressure is and how to really deviate the pressure," Bedford said during the press conference on Wednesday.

The announcement comes after Duffy said earlier this week that the FAA will be forced to shut down the airspace in some areas if the shutdown continues into next week.

United Airlines released a statement on the reduction, saying long-haul international and hub-to-hub flights will not be impacted.

The airline said customers looking for a refund on any flight can get one, even if flying basic economy or with a non-refundable ticket. The company will use its app, as well as email and push notifications to update customers, according to the statement.

The forthcoming reduction in capacity marks an unprecedented move by the FAA and the Department of Transportation. Bedford said he has never seen this happen before in his 35-year career.

Both Bedford and Duffy emphasized that the decision to cut down flights is a proactive measure based on data the department has reviewed, and the staffing pressures anticipated to grow during the shutdown. Duffy said the action is being taken to "reduce the risk profile in the national airspace."

Echoing him. Bedford added, "We're not going to wait for a safety problem to truly manifest itself when the early indicators are telling us we can take action today to prevent, you know, things from deteriorating. So the system is extremely safe today will be extremely safe tomorrow."

Bedford said the cuts to each airline's flight schedules will be proportionate, adding that the agency recognizes there's "no perfect solution."

The FAA said it will consider various factors when finalizing the cuts, including airlines that already operate a less-than-daily service at an airport.

Airlines for America, a trade association representing many major U.S. airlines, released a statement after the announcement, saying, "We are working with the federal government to understand all details of the new reduction mandate and will strive to mitigate impacts to passengers and shippers."

White House Press Secretary Karoline Leavitt told ABC News that President Donald Trump is aware of the FAA restrictions.

Full list of airports that will see cuts:

• Anchorage International

• Baltimore/Washington International

• Boston-Logan International

• Charlotte Douglas International

• Chicago Midway International

• Chicago O'Hare International

• Cincinnati/Northern Kentucky International

• Dallas-Fort Worth International

• Dallas Love Field

• Denver International

• Detroit Metropolitan Wayne County

• Fort Lauderdale-Hollywood International

• George Bush Houston Intercontinental

• Hartsfield-Jackson Atlanta International

• Honolulu International

• Houston Hobby

• Indianapolis International

• Las Vegas Harry Reid International

• Los Angeles International

• Louisville Muhammad Ali International

• Memphis International

• Miami International

• Minneapolis-Saint Paul International

• New York LaGuardia

• New York John F. Kennedy International

• Newark Liberty International

• Oakland San Francisco Bay International

• Ontario International

• Orlando International

• Philadelphia International

• Phoenix Sky Harbor International

• Portland International

• Ronald Reagan Washington National

• San Diego International

• Salt Lake City International

• San Francisco International

• Seattle-Tacoma International

• Tampa International

• Teterboro

• Washington Dulles International