U.S. flights were delayed while airports told passengers to check with their airlines for updates on Wednesday as the Federal Aviation Administration (FAA) scrambled to fix a system outage.

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.

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

According to Reuters, more than 760 flights were delayed within, into or out of the United States as of Wednesday 6:30 am ET (1130 GMT), flight tracking website FlightAware showed, without citing reasons.

Another 91 flights within, into or out of the country were also cancelled.

Shares of U.S. carriers fell during premarket trading on Wednesday.

Southwest Airlines was down 2.4%, while Delta Air Lines, United Airlines and American Airlines were down about 1%.

“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,” it said on Twitter.

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.

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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.

“Arriving and departing passengers can expect delays this morning and throughout the day,” Austin-Bergstrom International Airport said on Twitter, adding that there were ground stops across the country.

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

The Star