The US Federal Aviation Administration lifted a nationwide ground stop at airports Friday after a telecommunications outage prompted authorities to halt flights as a precaution.

The agency had issued the ground stop alert Friday evening following what Transportation Secretary Sean Duffy described as a "communication issue," triggering an hours-long suspension of flights across the US.

The suspension affected thousands of flights across the US before the aviation regulator lifted the alert.

Speaking at a news conference at the White House, Homeland Security Secretary Markwayne Mullin said the public had not been endangered during the incident.

"I just want to reiterate this: the public was never at danger through this process," Mullin told reporters.

"There was no flights at risk. FAA was able to handle it. We did it out of an abundance of caution," he added.

Asked whether a cyberattack could have caused the outage, Mullin said investigators were still examining the incident.

"We're still looking at it. We're not ruling out anything," he said.

Mullin suggested the outage may have resulted from new technology overwhelming older infrastructure during system upgrades.

"What it appears to be right now is, as Secretary Duffy and FAA was going in and putting patches into the system, they were putting new equipment and new technology, and that new technology was overwhelming the older system," he said.

'An emergency'

Duffy described the incident as "an emergency," saying he could not anticipate "every catastrophic failure" involving the aging air traffic control network.

He said smaller outages had occurred previously but that he had not expected a disruption of this scale.

Warning that further failures were likely without investment in infrastructure, Duffy urged Congress and the public to support rebuilding the system.

"You will have more of them unless the American people and the Congress tell us to build," he said.

Duffy claimed the equipment that failed was "old legacy equipment" and warned that "it's going to continue to fail."

He called for a "complete overhaul of the system," estimating that the project would cost about $50 billion.

"President Trump is a builder," Duffy said, arguing that the president could not carry out the overhaul without congressional funding. (Anadolu Agency)