United Airlines has been forced to ground their entire fleet due to a fire alarm at their operations centre.

The world's largest airline has said "flights may be temporarily delayed", and is recommending passengers use the United app for the most up-to-date flight information.

The US's Federal Aviation Administration (FAA) confirmed the news, saying in an advisory that the ground stop is in effect at all airports due to the fire alarm at the company's headquarters in the Willis Tower, 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.”