The Federal Aviation Agency (FAA) has lifted its order grounding all departing U.S. flights on Wednesday morning after a computer outage led to widespread delays. 
The agency said some systems are now back online, but that operations at the Notice to Air Missions System (NOTAM), which provides essential information to flight personnel, remain limited. Pilots are required to consult the NOTAM before taking off for any information on conditions that could impact the flight. 
"The FAA is working to restore its Notice to Air Missions System. We are performing final validation checks and reloading the system now," the agency tweeted. "Operations across the National Airspace System are affected."
The outage comes just two weeks after Southwest Airlines canceled and delayed the majority of its flights for several days, in part due to outdated IT systems. 
Secretary of Transportation Pete Buttigieg tweeted that he is in touch with the agency, and it's "working to resolve this issue swiftly and safely so that air traffic can resume normal operations, and will continue to provide updates."
United Airlines, meanwhile, tweeted that it has delayed all domestic flights:
Updated with FAA lifting grounding order.