U.S. flights were slowly beginning to resume departures after Federal Aviation Administration lifted its ground stop order after scrambling to fix a system outage overnight that affected thousands of flights.
The FAA had earlier it had ordered airlines to pause all domestic departures after its pilot alerting Notice to Air Missions system went out. The FAA said departures were resuming at Newark and Atlanta airports "due to air traffic congestion in those areas. We expect departures to resume at other airports at 9 a.m. ET."
Flights already in the air were allowed to continue to their destinations.
U.S. President Joe Biden has been briefed on the FAA system outage, press secretary Karine Jean-Pierre said on Twitter. There is no evidence of a cyber-attack at this point, but the president has asked for an investigation, she said.
The FAA said it was working to restore a system that alerts pilots to hazards and changes to airport facilities and procedures that had stopped processing updated information.
"The FAA is still working to fully restore the Notice to Air Missions (NOTAM) system following an outage. While some functions are beginning to come back on line, National Airspace System operations remain limited," the FAA said on Twitter.
A total of 32,578 flights were delayed within, into or out of the United States as of 8:07 a.m. ET (1255 GMT), flight tracking website FlightAware showed, without citing reasons. Another 409 within, into or out of the country were also canceled.
Shares of U.S. carriers fell in Wednesday's premarket trading. Southwest Airlines was down 2.4%, while Delta Air Lines Inc, United Airlines and American Airlines were down about 1%.
A NOTAM is a notice containing information essential to personnel concerned with flight operations, but not known far enough in advance to be publicized by other means.
Information can go up to 200 pages for long-haul international flights and may include items such as runway closures, bird hazard warnings and construction obstacles.
United Airlines said it had temporarily delayed all domestic flights and would issue an update when it learned more from the FAA.
Germany's Lufthansa and Air France both said they were continuing to operate flights to and from the United States, while the French airline said it was monitoring the situation. The operator of Paris international airports - Paris Charles de Gaulle airport and Orly airport - said it expects delays to flights.
Austin-Bergstrom International Airport said on Twitter that ground stops across the country were causing delays. A ground stop is an air traffic control measure that slows or halts aircraft at a given airport.
In an earlier advisory on its website, the FAA said its NOTAM system had "failed," although NOTAMs issued before the outage were still viewable. Earlier this month, a problem with a different airline computer control system delayed dozens of flights in Florida.
A total of 21,464 flights are scheduled to depart airports in the United States on Wednesday with a carrying capacity of nearly 2.9 million passengers, data from Cirium shows.
American Airlines has the most departures from U.S. airports with 4,819 flights scheduled, followed by Delta Air Lines and Southwest Airlines, Cirium data showed.