WASHINGTON: US flights were slowly beginning to resume departures and a ground stop was lifted after the Federal Aviation Administration (FAA) scrambled to fix a system outage overnight that had forced a halt to all US departing flights.

The cause of the problem with a pilot-alerting system, which delayed thousands of flights in the United States, was unclear, but US officials said they had so far found no evidence of a cyberattack.

The outage occurred at a historically slow time for US travel after the December holiday travel season, but airlines have said demand remains strong as travel continues to recover to near pre-pandemic levels.

“Normal air traffic operations are resuming gradually across the US following an overnight outage to the Notice to Air Missions system that provides safety info to flight crews. The ground stop has been lifted. We continue to look into the cause of the initial problem,” the FAA Tweeted.

The number of flights impacted continued to steadily rise even after the ground stop was canceled. One issue airlines are facing is trying to get planes in and out of crowded gates, which is causing further delays.

More than 5,400 flights had been delayed and 900 canceled according to the FlightAware website as officials said it will take hours to recover from the halt to flights.