The United States has lifted its order to ground all flights across the country after earlier communications problems at regional air traffic control centres.
The earlier notice from the Federal Aviation Administration said the ground stop was being imposed "due to extraordinary circumstances and for reasons of safety".
Planes already in the air were allowed to land, but those on the ground could not take off.
The FAA made clear there was no connection between the problem and the Microsoft outage that disrupted millions of users on Friday. While many are now able to access their Outlook and Teams, the issue is still being fixed.
US Transportation Secretary Sean Duffy said the FAA was investigating a communications issue affecting the 22 regional air traffic control centres that direct planes en route between airports nationwide.
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This was the first nationwide ground stop since January 2023, when a key FAA computer system malfunctioned. Before that the last time had been after the September 11 attacks.
Secretary of Homeland Security Markwayne Mullin said there was no indication yet that "this was a nefarious attack against us. It's just a telecommunications outage."
"Everybody's safe. There's no reason to be concerned about your loved ones if they're flying," Mullin said.
Mullin said investigators will look at whether there is any link to the FAA's nationwide rollout of a new artificial intelligence system to help identify problems and reroute planes.
The government has been working to overhaul the outdated technology that air traffic controllers rely on since US Congress approved US$12.5 billion ($18 billion) for the project last year.
Miles of old copper wires have been replaced with fibre optic lines and old radio systems are gradually being replaced at airports across the country.
But Duffy said that another US$30 billion ($44 billion) will be needed to complete the work, which Congress has to approve.
The problem appeared similar to a regional air traffic control communication outage in September that snarled hundreds of flights across the US northeast, aviation safety expert Steve Arroyo said, except this time on a nationwide scale. It may be linked to the outdated technology used by air traffic controllers.
"If you don't have that ability, whether it's because the old wires that they have are just run down or for whatever reason, you've got to stop airplanes on the ground," Arroyo, a former United Airlines pilot, said.
"And not put them in the air where you're going to risk not being able to communicate with them."
The US National Weather Service also said on Saturday it was experiencing nationwide internet and communications issues. There was no immediate indication the two incidents were connected.
