The Federal Aviation Administration ordered a nationwide grounding of all departing flights across the United States on Friday after a communication outage disrupted air traffic control frequencies and radar capabilities at centres nationwide. The FAA said it acted "due to extraordinary circumstances and for reasons of safety" until further notice, according to NBC News and CNN.

By 5:20 p.m. Eastern Time, the disruption had already produced more than 9,000 delayed flights within, into or out of the United States, with 440 cancellations, according to flight tracking website FlightAware.

What caused the ground stop?

The restriction followed a communication issue affecting all Air Route Traffic Control Centres, according to a statement by US Secretary of Transportation Sean Duffy posted on X, to which the FAA directed all inquiries.

"FAA technicians are investigating the issue. We will provide immediate updates as we have them," Duffy said.

The FAA command centre told airlines it had no immediate information on either the cause or the expected duration of the outage, according to reports. The disruption affected both air traffic control frequencies, the radio channels controllers use to communicate with pilots, and radar capabilities used to track aircraft in flight.

Was it a security threat?

Homeland Security Secretary Markwayne Mullin said there was no indication the outage stemmed from a hostile act. Speaking to reporters at the White House, Mullin said: "There is no indication that we've had, no reason to believe, that as of right now this was a threat to the United States."

He later characterised the incident as a telecommunications outage, according to CNN.

What is the operational picture?

The FAA said every flight affected by the disruption had been accounted for. The agency added that international flights would be handled on a case-by-case basis, leaving open the possibility that some inbound and outbound long-haul operations could continue while domestic departures remained frozen.

The scale of the disruption, measured in a single afternoon:

• More than 9,000 flights delayed within, into or out of the United States as of 5:20 p.m. Eastern Time
• 440 flights cancelled
• All Air Route Traffic Control Centres affected by the communication failure
• No immediate timetable for restoration provided by the FAA

What comes next?

The ground stop remains in effect until further notice, and the FAA has committed to updates as its technicians work through the investigation. With no identified cause and no projected duration, airlines face an open-ended disruption heading into the weekend.