All US domestic flights have been grounded for several hours because of a glitch with the flight control system.
The Federal Aviation Administration (FAA) says there is a problem with the system that alerts pilots to potential hazards on flight routes.
It is working to restore it but says no flights will take off until at least 0900 ET (1400 GMT).
In a statement, the FAA said some functions were beginning to come back online, and would give updates later.
It was not immediately clear if the outage would impact international flights.
US President Joe Biden has been briefed about the outage, and the White House said there was no evidence of a cyberattack “at this point”.
In a tweet, the White House Press Secretary said the President had called for a “full investigation into the causes”.
The FAA said the problem lies with its Notice to Air Missions System.
American Airlines said the outage was impacting “all flights including all carriers”.
United Airlines said they were delaying all domestic flights until they heard more from the FAA.
Texas’s Austin-Bergstrom International Airport confirmed in a tweet that “a FAA outage is causing ground stops at AUS and other airports across the country.”
It said arriving and departing passengers should expect delays.
US Department of Transportation Secretary Pete Buttigieg said he had been in contact with the FAA on Wednesday morning, and that they were working to resolve the issue “swiftly and safely”.
Passengers have posted on social media that they are experiencing delays.
“Always great to get up at 0400 to be at the gate in plenty of time for your flight that is now delayed by 37 minutes and counting,” said Daniel Huard in a Facebook post. “This will be a long day of travel.”
Michael Remy arrived to an airport in Virginia at 0600 ET (1100 GMT) planning to head to North Carolina for vacation. His flight was delayed right before boarding. “It is what is, so, you can only get so upset,” he told the BBC. “I may have seen it differently if I was headed to a wedding or a funeral, though.”