NEW YORK, United States (AFP) — All flights in the US were grounded Friday, sparking travel chaos across the vast country, after what government officials said was a major communications outage, insisting it was not the result of any “nefarious attack”.

The nationwide stop left tens of thousands of passengers in limbo, with some pulled off planes moments before departure by airline staff who appeared in the dark about unfolding events.

The incident marked only the second time such an alert was given in US airspace since the 9/11 attacks.

“So supposedly, nobody’s flying, but they haven’t told the public yet,” said an exasperated Joseph Anderson, 72, who was trying to fly from Las Vegas to Oakland, California.

“We’re trying to debate whether or not we need to reschedule or drive or what, but if the whole country is stopped, that means the highways are going to be jammed too.”

The Federal Aviation Administration (FAA) “issued a nationwide ground stop for all US airports following a communication issue impacting all air route traffic control centres,” the agency said. “FAA technicians are investigating the issue.”

In a subsequent message on X, the agency described the problem as “a temporary telecommunications outage” and said it was acting “out of an abundance of caution”.

Homeland Security Secretary Markwayne Mullin said there was no sign of a threat to the United States.

“This isn’t another 9/11,” he said.

“There was no indication, as of yet, that this was a nefarious attack against us, it’s just a telecommunication outage, as of right now,“” Mullin said, adding officials were “not ruling out anything at this point”.

There was a short nationwide stop in January 2023 when a pilot notification system failed, but the only other national stop happened in the wake of the 9/11 attacks, when no aircraft took to the skies for days.

Friday’s FAA order came a day after US Transportation Secretary Sean Duffy formally asked Congress for US$30 billion to upgrade and modernise the US Air Traffic Control System.

“This investment is critical to complete the modernization of our national airspace — delivering a resilient telecommunications backbone for air traffic controllers, modern hardware and software, state-of-the-art control towers, and world-class airport infrastructure,” Duffy and Office of Management and Budget Director Russell Vought wrote in a letter to lawmakers.

It also came the same day that Microsoft apologised for a global outage that wreaked havoc for millions of users, public services and businesses worldwide.

The company ruled out any malicious “cyber or AI” activity as the cause of the disruption, which affected several Microsoft products from Thursday. Most operations had resumed by late Friday.