Washington : An unprecedented outage of a key system forced the United States to ground all departures for hours on Wednesday, leading to delays and cancellations that affected at least 8,700 flights and millions of passengers following a roughly two-and-a-half hour halt that triggered ripple effects lasting throughout the day.

Close to 1400 GMT (7.30pm IST), the Federal Aviation Administration (FAA) said that normal operations were resuming gradually, but airlines warned of lingering delays.

“Normal air traffic operations are resuming gradually across the US,” the FAA said on Twitter. “The ground stop has been lifted. We continue to look into the cause of the initial problem.”

The outage led to more than 7,539 flights being delayed and 1,163 flights being cancelled, data from flight tracking website Flight Aware showed at 11.30pm (IST).

The FAA said it had identified a problem with the Notice to Air Missions system (Notam), which provides information to flight crews about hazards, changes to airport facilities and other essential information. The issue was first spotted late on Tuesday, but the disruptions were limited since those are hours for the few red-eye services.

US President Joe Biden was briefed on the situation by secretary of transportation Pete Buttigieg, the White House said on Wednesday in a tweet. The President ordered a full investigation, but there was “no evidence of a cyberattack at this point”, the White House said.

The massive system failure is one of the most significant in recent decades for the FAA and comes after a year of tests of the nation’s air system as the return from Covid-19 stressed airlines and air-traffic operations.

The grounding potentially could affect more than a million passengers who were booked on 19,621 domestic flights across various airlines, according to aviation data provider Cirium. The numbers do not include incoming international flights.

According to news agency Bloomberg, Notam, the computer system that shares the notices to pilots, airlines and other users began developed problems late on Tuesday night and had to be completely taken down in the early hours of Wednesday, said a person familiar with FAA’s actions, who asked not to be identified. FAA technical workers have been troubleshooting the system since then, the person said.

There was no indication that the outage affected the FAA’s ability to track and guide aircraft, or that it caused any safety incidents. Typically, in such scenarios, critical communications can still be carried out via Air Traffic Control (ATC) communications, but this could be challenging if the Notam, which is automated, is down.

By early Wednesday, passengers due to fly domestically began reporting delays on social media, and United said it would temporarily ground flights.

Near 1430 GMT, a screen at Reagan Washington National Airport was overwhelmingly red with flight delays and just a handful of departures.

“Customers may continue to see some delays and cancellations as we work to restore our schedule,” United Airlines said shortly after the FAA stop order was lifted, adding it would refund customers who no longer wish to travel.

Speaking to reporters, Biden said that “aircraft can still land safely, just not take off right now.”

“They don’t know what the cause of it is, they expect in a couple of hours they’ll have a good sense of what caused it and will respond at that time,” Biden said.

Buttigieg directed “an after-action process to determine root causes and recommend next steps,” he said on Twitter.

The halt comes in the wake of a large-scale aviation meltdown in the United States over the Christmas holiday, as a storm brought unseasonably cold temperatures to the majority of the country and caused chaos, with thousands of flights delayed or cancelled.

Hard-hit Southwest Airlines cancelled more than 15,000 flights over eight days after what it said was a breakdown in its scheduling systems.