Domestic flights across the US were temporarily grounded on Wednesday morning, after an IT failure in a critical aviation safety system.

The Federal Aviation Administration (FAA) said that the system that alerts pilots and airlines about any hazards was not functioning. The breakdown led to more than 7,800 flights being delayed and 1,200 being canceled , the flight tracking website FlightAware showed.

The ground stop was lifted at 9am with the FAA declaring that operations were “resuming gradually across the United States”, but travelers were still left facing another chaotic day of air travel following severe disruptions over the holiday period.

“They don’t know what the cause is,” Joe Biden told reporters after speaking to the transportation secretary, Pete Buttigieg. “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.”

The White House said there was no evidence of a cyber-attack but the causes of the IT failure would be investigated in full by the Department of Transportation.

International US-bound flights were continuing to take off from Europe and elsewhere.

The aviation regulator said its Notam (Notice to Air Missions) system had “failed” and it was working to restore it.

It said: “While some functions are beginning to come back on line, National Airspace System operations remain limited.”

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

Jordan Cousins, 25, on his way to Nashville on Southwest Airlines from New York’s LaGuardia, said his flight had been delayed twice and then canceled entirely.

“I’ve been here since 7am and this pushes back everything I was trying to do. First I thought it was a cyber-attack, but they said it was some kind of malfunction. So I had all sorts of curiosities,” he said.

Crowley said he had noticed that US air travel had become precarious.

“Travelling is coming a bit of a hassle. It’s this and then it’s that. You never know. You may have a smooth flight or there may be a problem. It may be at the counter, with the plane, or something,” he said. “Plans never go as planned.”

The White House press secretary, Karine Jean-Pierre, said: “There is no evidence of a cyber-attack at this point, but the president directed [the Department of Transportation] to conduct a full investigation into the causes. The FAA will provide regular updates.”

Wednesday’s chaos came after a troubled holiday season for air travelers. Bad weather led to the cancellation of thousands of flights, a situation compounded by issues at Southwest Airlines that led to the cancellation of thousands more.

More than 20,000 flights were scheduled to depart airports in the US on Wednesday, according to aviation analytics firm Cirium, with almost 2.9m seats.

The delays affected carriers around the US. American Airlines, the biggest carrier by volume, said it was working with the FAA to minimise disruption. United Airlines said it had paused all domestic flights.

A Notam is a notice containing information essential to personnel concerned with flight operations, but not known far enough in advance to be publicised by other means.

Information can go up to 200 pages for long-haul international flights and may include items such as runway closures, general bird hazard warnings or low-altitude construction obstacles.