United Airlines was forced to ground its entire fleet late on Thursday after a fire alarm at their operations centre.
The world's largest airline said "flights may be temporarily delayed".
The US's Federal Aviation Administration (FAA) confirmed the news, saying in an advisory that the ground stop was in effect at all airports due to the fire alarm at the company's headquarters in the Willis Tower, Chicago.
A later air traffic advisory notice shortly after 1am UK time confirmed the nationwide ground stop had been cancelled.
A spokesman for the airline said: "A fire alarm sounded at our operations centre, which caused employees to move to our nearby backup facility and resulted in a brief nationwide ground stop of United aircraft.
"No flights were diverted, United Express regional flights [were] not affected."
Passengers who were already in the air at the time of the ground stop were warned they could "expect some airborne holding due to excessive demand".
Footage from ABC News saw lined up planes waiting to depart from San Francisco's International Airport taking off in quick succession once the ground stop was lifted.
FAA
|The US's Federal Aviation Administration (FAA) confirmed the news, saying in an advisory that the ground stop was in effect at all airports due to the fire alarm at the company's headquarters in Chicago
One flyer affected by the chaos, Joe Tegtmeyer, said on social media: “Well, I’m trying to get to California but United Airlines have been battling a major disruption in service at Denver due to thunderstorms and now, a nation-wide stop service due to a major fire at their headquarters.
“So, I’m stuck in San Antonio with no way yet to get to San Francisco today. Trying to figure out a plan.”
Another traveller, Justin Guzman, added: “Our United Airlines flight at [Washington] Dulles is returning to the gate due to a fire in their Chicago headquarters.
“Not sure how long we will be grounded.”
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GETTY | United Airlines was forced to ground its entire fleet late on Thursday after a fire alarm at their operations centre
The incident came just days after a similar ground stop was issued by Alaska Airlines, leading to hours of delays on Sunday night.
In that case the ground stop was due to a system-wide IT outage, and all mainline flights across the United States were grounded for roughly three hours, with delays continuing into Monday morning.
Despite the airline confirming the issue was not caused by a cyberattack, concerns have been raised over the fragility of aviation regarding digital disruption.
In June, the FBI warned that America's airlines are under active cyber threat.
PA | Queues at Gatwick Airport during the delays last year
Last year, more than 5,000 flights were cancelled worldwide after Microsoft Windows was affected.
Passengers described scenes in UK airports as "bedlam" as delays became widespread and queues piled up at check-in desks.
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