The FAA had said in a tweet just before 7:20 a.m. that it was ordering airlines to pause all domestic departures until 9 a.m. ET “to allow the agency to validate the integrity of flight and safety information" as it worked to restore the NOTAM system.

In subsequent updates, it said that all flights in the sky were safe to land.

“Pilots check the NOTAM system before they fly. A Notice to Air Missions alerts pilots about closed runways, equipment outages, and other potential hazards along a flight route or at a location that could affect the flight,” the FAA said.

White House press secretary Karine Jean-Pierre said in a tweet that there was "no evidence of a cyberattack at this point," and added that President Joe Biden had directed the Department of Transportation to conduct a full investigation into the causes. She added: “The FAA will provide regular updates.”

A senior law enforcement official told NBC News the FBI was seeing no evidence that a cyberattack caused the outage. Cyber security experts say the most common cause of problems like this is a bad software update. The cause of Wednesday's incident is still unclear, however.

“Today’s FAA catastrophic system failure is a clear sign that America’s transportation network desperately needs significant upgrades," U.S. Travel Association President and CEO Geoff Freeman said in a statement Wednesday. "Americans deserve an end-to-end travel experience that is seamless and secure. And our nation’s economy depends on a best-in-class air travel system.

"We call on federal policymakers to modernize our vital air travel infrastructure to ensure our systems are able to meet demand safely and efficiently," he said.

Transportation Secretary Pete Buttigieg said in a tweet that he had "been in touch with FAA this morning about an outage affecting a key system for providing safety information to pilots."

United Airlines said earlier it had temporarily delayed all domestic flights. It said it would issue an update when it learned more from the FAA about the situation.

Southwest Airlines said it was "closely monitoring" the situation and that it "may impact the start of operations" Wednesday.

"An FAA system outage is causing ground stops at AUS and other airports across the country," the Austin-Bergstrom International Airport said in a tweet.

"Arriving & departing passengers can expect delays this morning & through the day," it said, adding: "Please stay in contact with your airline & check your flight status before heading to AUS."

The news came after a number of social media users said they had been affected by the situation.

Heather Allen, 32, was meant to fly from New York City to Seattle with her fiancé to visit her family for a delayed holiday visit. She was watching a movie on her plane and still on the tarmac at John F. Kennedy International Airport when she and other passengers were told to get off their Delta Airlines flight.

She said she learned of the outage by reading the news on Twitter and had been on the plane for about an hour before she had to deplane.

“Trying to be patient, but feeling frustrated,” Allen said. She said the situation at the airport was “not currently chaotic, but could be if delays are longer.”

The issue also appeared to have affected some flights to the U.S.

A number of airports outside the U.S. said operations were continuing as normal, but the international airport operator Aéroports de Paris, or Airports of Paris, said all flights by American airline companies had been delayed. It said non-American airlines were flying out as normal without interruption.

Air France said all of its U.S.-bound flights were operating as planned and were not affected by the FAA computer outage. It said it continued to monitor the situation.

"As far as we are aware, we are still operating to/from the U.S. at the moment," a spokesperson for Gatwick Airport in London said.

A spokesperson for Frankfurt Airport in Germany said the FAA outage had not affected its operations.