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 "issued a nationwide ground stop for all US airports following a communication issue impacting all Air Route Traffic Control Centers," 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 $30 billion to upgrade and modernize 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 apologized 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.

At New York's LaGuardia Airport, Tiara Johnson said she had been trying to get to Orlando for a concert with a friend.

“They just keep delaying it... So it started out at 7:55. Then it went to 8:30, 9:30. Now it's at 10:30. So we don't know when it's gonna leave," she told AFP.

“I feel very frustrated because you know, you come to the airport and then all of a sudden your whole plans have changed and you don't know when you're leaving, We're kind of stuck."

At Harry Reid International Airport in Las Vegas, airline staff gave conflicting information about the length of the ground stop, with one suggesting eight hours and another saying it could be 24.

Travelers were left in the dark, with some saying they had been told their flight was delayed because of wind, while others were left to gather what information they could.

Boards at the airport continued to show departures leaving as normal, while mobile boarding passes held by those stuck on the ground appeared to show their flight on its way.

Stranded passengers were scrambling to find alternatives, with long lines forming at rental car counters at the airport in Las Vegas.

Susan Sardina said she had boarded her flight back to Orange County, California when staff told passengers there was a problem.

"We were already on our flight to leave and we were told to get off," she told AFP as she stood in a lengthy queue for a rental car.

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