(Reuters) -Top U.S. airlines including Delta, United and American issued ground stops on Friday citing communication issues, as a global outage roiled operations across a wide swathe of industries around the world.

American Airlines, however, later said in a statement it had re-established operations. Frontier and Spirit too cancelled directives to ground planes.

It was not clear if the groundings reported by the major U.S. airlines were related to outages at Microsoft and cybersecurity firm Crowdstrike that affected banking, healthcare and a number of other sectors.

Smaller players Frontier Airlines, Allegiant and SunCountry had earlier reported outages.

While American, Delta and United, the top three U.S. airlines, had earlier said a technology issue with a third-party vendor impacted them without naming the vendor, Frontier said that a “major Microsoft technical outage” hit its operations temporarily.

“The Allegiant website is currently unavailable due to the Microsoft Azure issue,” Allegiant said in a statement to CNN. It did not respond to Reuters’ request for a comment.

The U.S. Federal Aviation Administration said in an emailed statement it was closely monitoring the technical issue impacting IT systems at U.S. airlines and that several airlines had requested its assistance with ground stops.

U.S. Secretary of Transportation Pete Buttigieg said the department was monitoring flight cancellation and delay issues at Frontier, adding that it will hold the company and all other airlines “to their responsibilities to meet the needs of passengers”.

A total of 518 flights were cancelled, with over 720 delayed, as of 5:58 am ET on Friday, according to data tracker FlightAware.

Microsoft said its outage started at about 6 pm ET on Thursday, with a subset of its customers experiencing issues with multiple Azure services in the Central U.S. region.

Azure is a cloud computing platform that provides services for building, deploying, and managing applications and services.

Separately, Microsoft said it was investigating an issue impacting various Microsoft 365 apps and services.

(Reporting by Maria Ponnezhath and Shivani Tanna; Additional reporting by Chandni Shah, Nathan Gomes, Surbhi Mishra, Angela Christy and David Shepardson; writing by Abinaya Vijayaraghavan and Shubham Kalia; Editing by Mrigank Dhaniwala, Varun H K and Nivedita Bhattacharjee)

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