The Federal Aviation Administration (FAA) has issued a nationwide ground stop for all US airports because of a communication issue affecting all air route traffic control centres, US Transportation Secretary Sean Duffy said.

FAA technicians are investigating the issue, Duffy added on Friday.

"Traffic should remain ground stopped until further notice," the FAA said in an advisory. "International flights will be managed on a case by case basis."

The last nationwide ground stop occurred in January 2023 due to a pilot alert system outage. Before that, the measure had not been implemented since the September 11, 2001 attacks.

Nearly 7,000 US flights have been delayed or cancelled due to nationwide groundstop, tracker FlightAware said.

The US government has been working to overhaul the outdated technology that air traffic controllers rely on since Congress approved $12.5 billion for the project last year.

Miles of old copper wires have been replaced with fiber optic lines and old radio systems are gradually being replaced at airports across the country.

But much work remains.

Duffy has said that another $30 billion will be needed to complete the work, and the White House has begun lobbying Congress to approve it.

'No indication as of yet that this was a nefarious attack'

"There has been a temporary telecommunications outage - out of an abundance of caution all flights have been halted. Every flight has been accounted for and international flights will be handled on a case by case basis," the agency said.

Separately, Homeland Security Secretary Markwayne Mullin told the reporters that there was "no indication as of yet that this was a nefarious attack."

"There was no indication as of yet that this was a nefarious attack. It is just a telecommunication outage as of right now. Everybody's safe. There is no reason to be concerned about your loved ones if they're flying or anything like that," Mullin said.

Mullin said there is "no reason" for the public to be afraid or scared.

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

But asked if a cyberattack could eventually be found to be responsible, Mullin said officials were "not ruling out anything at this point."

The homeland security chief said however that he did not believe the air traffic stop was linked to a huge global Microsoft outage that affected public services around the world.

"I don't see any reason to connect those two at all. Those are separate issues," Mullin said.

Mullin added that "every flight has been accounted for" and said incoming international flights would be handled on a "case by case basis."

But he said the situation could "take some time" to be fixed. "This isn’t going to just be resolved and turned around within the hour," he added.