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.