Washington - As a result of a massive nationwide technical glitch, about 10 000 flights in and out of the US were delayed, while more than 1 300 others were cancelled, according to the country's Federal Aviation Administration (FAA).
Wednesday's disruption was due to a "damaged database file", the BBC quoted the FAA as saying, adding that "at this time, there is no evidence of a cyber attack".
Although normal air traffic operations were slowly resuming, further delays are expected to continue through at least Thursday and possibly longer as airlines try to get planes in and out of crowded gates.
Airports nationwide were affected, from Denver to Atlanta to New York City.
The technical issues marked the first time in nearly two decades that flights across the US were grounded.
President Joe Biden has called for a "full investigation", the White House press secretary said.
Meanwhile, Department of Transportation Secretary Pete Buttigieg told CNN that the FAA had grounded flights out of "an abundance of caution" after it noticed irregularities with its Notice to Air Missions System, which provides real-time safety information to pilots "about closed runways, equipment outages, and other potential hazards along a flight route or at a location that could affect the flight".
"My primary interest, now that we've gotten through the immediate disruptions of the morning, is understanding exactly how this was possible and what steps are needed to make sure it doesn't happen again," Buttigieg said.
Major US airlines said they were still closely monitoring the situation.
While American Airlines said it was working with the FAA to minimise customer disruption, United Airlines said it would waive change fees and any difference in fare for customers rescheduling flights departing on or before January 16, the BBC reported.
Delta said it was "safely focused on managing our operation during this morning's FAA ground stop for all carriers", adding it would provide updates as soon as possible.
For international passengers, Air Canada said the outage would impact on cross-border operations on Wednesday, but it couldn't initially say to what degree.
The flag carrier said it would put in place a "goodwill policy" so affected passengers can change their travel plans.
Meanwhile, airports in Paris - Charles de Gaulle and Orly - said they expected delays to US flights, and Air France said it was monitoring the situation.
For UK passengers, British Airways said its flights to and from the US would operate as planned, and Virgin Atlantic said it was continuing to operate its schedule of US flights departing from the UK.
However, some US departures, the airline said, might be affected by delays.
Germany's Lufthansa and Spain's Iberia said they were still operating flights to and from the US normally for now.