The US Federal Aviation Administration (FAA) issued a rare nationwide ground stop for all departing flights on Friday following a communications outage affecting air traffic control centers, disrupting air travel and causing confusion among passengers.

The agency said it had "issued a nationwide ground stop for all US airports following a communication issue impacting all Air Route Traffic Control Centers," adding that "FAA technicians are investigating the issue."

While the FAA has issued similar orders in recent years affecting individual airlines because of communications problems, such restrictions have generally lasted a few hours or less. Nationwide ground stops affecting the entire system are considerably less common.

In January 2023, the agency ordered a nationwide ground stop following an outage affecting its Notice to Air Missions (NOTAM) system, which provides pilots with information about hazards, changes to airport facilities, and other developments that could affect flights. The disruption lasted just under two hours and was believed at the time to be the first nationwide grounding since the September 11 attacks.

US air traffic control system faces modernization push

Friday's disruption came a day after US Transportation Secretary Sean Duffy formally requested $30.1 billion from Congress to upgrade and modernize the country's 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.

The disruption also affected travelers at individual airports. At Las Vegas airport, the Southwest Airlines check-in desk informed passengers that no flights would take off for 24 hours.

Homeland Security chief rules out immediate indication of threat

Meanwhile, US Homeland Security Secretary Markwayne Mullin said there was no indication that the country was facing a threat following the nationwide ground stop, although authorities were continuing to investigate the cause of the disruption.

Speaking to reporters at the White House after meeting President Donald Trump, Mullin said officials had not ruled out the possibility of a cyberattack.

"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. "This isn't another 9/11. I just can't repeat that enough."

Asked whether a cyberattack could eventually be identified as the cause, Mullin said authorities were "not ruling out anything at this point."