Transportation Secretary Sean Duffy told a late-night briefing at the White House that US airspace was now "opened up," while warning it would take time for the travel chaos to clear.

"At 8:15 tonight Eastern Time, we've opened up the airspace," Duffy said. "Now does that mean every flight is going to fly right now? It's going to be slow."

The breakdown came a day after Duffy and the White House had formally asked Congress for US$30 billion to upgrade and modernize the US Air Traffic Control System.

"Our switches, our routers actually got overloaded with data, and they went down," Duffy said. "It's the old hardware that went down, and I think the question for the American people is: Do you want your Congress to fund this?"

Duffy however insisted it was still safe for Americans to fly.

Homeland Security Secretary Markwayne Mullin dismissed speculation about a coordinated cyber assault on the United States, or any link to the Microsoft outage that affected services worldwide.

"Welcome to Friday the 13th, right? You know, I want to make sure everybody understands that there is no connection between this and Microsoft. I know a lot of people are making that jump," Mullin said.

Mullin added there was "actually no cyber attack”.

"When we start looking at cyber threats, cyber attacks, typically some people are going to raise their hand up and say, 'Hey, we did that.' There's no chatter.' We haven't seen any footprints," he added.

But he said US security and law enforcement agencies were still checking and had ruled nothing out.

The two officials also defended President Donald Trump, who had yet to comment on the air stoppage some five hours after it began.

"President Trump immediately had people in his office," Mullin said. "We were communicating."

With almost 6,000 flights canceled including some international routes according to FlightAware, the recovery from the stoppage was expected to take some time.

Footage from airports around the country showed gates full, with planes waiting on taxiways.

Passengers thronged concourses, desperate for information about an outage that had thrown their travel plans into chaos.

US airspace is among the busiest in the world, and is ramping up to its most frenetic as the Thanksgiving holiday moves into view.

The nationwide ground stop marked only the second time such an alert had been issued in US airspace since the 9/11 attacks.

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