A computer outage at the Federal Aviation Administration has brought flights to a standstill across the U.S., with hundreds of delays quickly cascading through the system at airports nationwide.

The FAA ordered all U.S. flights to delay departures until 9 a.m. Eastern Time, though airlines said they were aware of the situation and had already begun grounding flights.

At 7:30 a.m. there were more than 1,200 delayed flights within, into or out of the U.S., according to the flight tracking website FlightAware. More than 100 have been cancelled.

Most delays were concentrated along the East Coast, but were beginning to spread west.

“The FAA is still working to fully restore the Notice to Air Missions system following an outage,” the FAA said in its latest Twitter update. “The FAA has ordered airlines to pause all domestic departures until 9 a.m. Eastern Time to allow the agency to validate the integrity of flight and safety information. Operations across the National Airspace System are affected.”

Transportation Secretary Pete Buttigieg said in a tweet that he is in touch with the FAA and monitoring the situation.

United Airlines said that it had temporarily delayed all domestic flights and would issue an update once it learned more from the FAA.

The FAA is working to restore what is known as the Notice to Air Missions System.

NOTAMs used to be available through a hotline but that was phased out with the internet. The alerts span from mundane information about construction at airports to urgent flight restrictions or broken equipment.

There is a potential for widespread disruption because of the outage. All aircraft are required to route through the system, including commercial and military flights.

European flights into the U.S. appeared to be largely unaffected.

The FAA said that it would provide frequent updates as it made progress.