NEW YORK (AP) — A computer outage at the Federal Aviation Administration brought flights to a standstill across the U.S. on Wednesday, 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, though airlines said they were aware of the situation and had already begun grounding flights.

At 7:30 a.m. Eastern, there were more than 1,200 delayed flights within, into or out of the United States, 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. Inbound international flights are into Miami International Airport continued to land, but all departures have been delayed since 6:30 a.m., said airport spokesman Greg Chin.

The FAA said it was working on restoring its Notice to Air Missions System.

“We are performing final validation checks and reloading the system now,” the FAA said. “Operations across the National Airspace System are affected.”

The agency said that some functions are beginning to come back on line, but that “National Airspace System operations remain limited.”

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.

Before commencing a flight, pilots are required to consult NOTAMs, or Notices to Air Missions, which list potential adverse impacts on flights, from runway construction to the potential for icing. The system used to be telephone-based, with pilots calling dedicated flight service stations for the information, but has now moved online.

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.

Irish carrier Aer Lingus said services to the U.S. continue, and Dublin Airport’s website showed that its flights to Newark, Boston, Chicago and Los Angeles were running on schedule.

“Aer Lingus plan to operate all transatlantic flights as scheduled today,” the carrier said in a prepared statement. “We will continue to monitor but we do not anticipate any disruption to our services arising from the technical issue in the United States.”

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

By Michelle Chapman