United Airlines had to initiate an urgent halt on its entire mainline fleet Thursday night following a fire alarm at its headquarters.

This decision raised concerns that it might lead to travel disruptions across the country, just days after a similar incident with Alaska Airlines led to extensive delays for travelers throughout the United States.

Passengers who were already in the air at the time of the ground stop were warned to ‘expect some airborne holding due to excessive demand.’

The ground stop has now been revoked, and a United representative stated: “A fire alarm was triggered at our operations center, prompting staff to relocate to our nearby backup facility, which briefly resulted in a nationwide halt of United aircraft operations.”

‘No flights were diverted, United Express regional flights [were] not affected.’ 

United operates the world’s largest fleet with more than 1,000 planes, all of which were temporarily placed on hold.

The order came just days after Alaska Airlines issued a similar ground stop which sparked hours of delays on Sunday night.

In that case, the airline issued requested the US Federal Aviation Administration stop all mainline flights across the United States due to a system-wide IT outage.

Planes were grounded from around 8pm PT (11pm ET) before operations resumed again at 11pm PT (2am ET), Alaska Airlines said in a statement.

The firm has as many as 238 planes in its fleet – all of which are Boeing aircrafts – and its biggest hubs include Seattle, Portland, LA and San Francisco. 

Shortly after the initial request, a secondary order came through to extend the ground stop to subsidiary airline Horizon Air, which operates another 45 planes.

Alaska Airlines admitted the ground stop would cause delays for travelers and have knock on effects into Monday.

‘At approximately 8 p.m. Pacific on Sunday, Alaska Airlines experienced an IT outage that’s impacting our operations,’ the airline said in a statement Sunday evening. 

‘We requested a temporary, system-wide ground stop for Alaska and Horizon Air flights until the issue is resolved.’ 

The airline has warned of ‘residual impacts to our operation throughout the evening’ amid concerns about widespread travel delays.

‘If you are traveling tonight, please check the status of your flight before leaving for the airport.

‘We apologize to our guests for this inconvenience.’ 

One passenger revealed he and his fiancé spent two hours waiting on a tarmac in Denver before the flight crew finally instructed them that the plane would not be taking off.

At 10.30pm, all passengers were instructed to disembark and await further instruction, he said.  

Another passenger, Riley Davis, said after two and a half hours on the tarmac, he still had no indication of when he would be free to deplane.

‘Since they cannot coordinate exact gates, they are manually assigning gates via radio communication,’ he revealed.

‘I would love to know when I can get some dinner, I’m starving.’

He was inundated with comments from other travelers who said they’d noticed tarmacs around the country blocked by Alaska Airlines planes which had nowhere to go.