FAA ground stop live updates: 'Zero evidence of anything nefarious,' FAA head says

Transportation Secretary Sean Duffy said operations are "slowly" returning to normal on Saturday with planes pushing back, taxiing and taking off.

"You will continue to see some delays and cancellations as carriers reschedule flights and reposition aircraft/crews where they are needed most," Duffy said in a post on X.

"Our top priority right now is making sure travelers get home safely to their families as quickly as possible," Duffy said.

Duffy also thanked air traffic controllers, pilots, and flight crews who have been working around the clock.

"The nation thanks you for your professionalism and dedication to the flying public," Duffy said.

12,506 flights have been cancelled so far between yesterday and today, per Flight Aware. The number is expected to grow throughout the day.

On Saturday morning already, 3,374 flights have been canceled and 1,008 are delayed nationwide, per FlightAware.

The total for Friday is 9,132 cancellations and 7,030 delays.

-ABC News' Clara McMichael

At a White House news conference, Transportation Secretary Sean Duffy insisted that the U.S. has the "safest airspace in the world" but also suggested an incident like Friday's ground stop would happen again and that flying will continue being “less safe” unless Congress provides funding for flight system advancement.

"We didn't think we would have this kind of outage," Duffy said. "The systems that were deployed kept everybody safe -- everyone landed. We're able to review what went wrong and then reboot the system and go. But you will have more of these. You will have more of them unless the American people and the Congress tell us to build."

Duffy called on Congress to provide $50 billion for a "complete overhaul" of the system.

Following the lifting of the ground stop, United Airlines said it is "resuming operations, including some planned Friday night flights."