US flights were grounded or delayed on Wednesday as the Federal Aviation Administration (FAA) scrambled to fix a system outage, with passengers told to check with airlines for updates.

The FAA said it had ordered airlines to pause all domestic departures until 9am. ET (1400 GMT) to allow the agency to validate the integrity of flight and safety information.

Flights already in the air were allowed to continue to their destinations. 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 1,230 flights were delayed within, into or out of the United States as of 7.19 am ET (1219 GMT), flight tracking website FlightAware showed, without citing reasons. Another 103 within, into or out of the country were also cancelled.

Shares of US carriers fell in Wednesday’s premarket trading. Southwest Airlines was down 2.4%, while Delta Air Lines Inc (DAL.N), United Airlines (UAL.O) and American Airlines (AAL.O) 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 (UAL.O) 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.

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.

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 US  airports with 4,819 flights scheduled, followed by Delta Air Lines and Southwest Airlines, Cirium data showed.