Major US carriers including American Airlines, Delta Airlines and United Airlines issued ground stops on Friday morning citing communication issues, less than an hour after Microsoft resolved its cloud services outage that impacted several low-cost carriers.

It was not immediately clear whether the call to keep flights from taking off were related to the earlier Microsoft cloud outage. Apart from American and Delta, UAL and Allegiant Air too grounded flights.

The FAA did not immediately respond to Reuters' request for comment.

Low-cost carriers Frontier Airlines, a unit of Frontier Group Holdings, Allegiant and SunCountry had earlier reported outages that affected operations. Frontier said late Thursday that it was in the process of resuming normal operations, and that the ground stop had been lifted.

Frontier said earlier that a "major Microsoft technical outage" hit its operations temporarily, while SunCountry said a third-party vendor affected its booking and check-in facilities, without naming the company.

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

"The Allegiant website is currently unavailable due to the Microsoft Azure issue," Nevada-based Allegiant said in a statement to CNN. Allegiant did not immediately respond to Reuters' request for a comment.

Frontier cancelled 147 flights on Thursday and delayed 212 others, according to data tracker FlightAware. 45% of Allegiant aircrafts were delayed, while Sun Country delayed 23% flights, the data showed. The companies did not give details on the number of flights impacted.

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 US 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. (Reuters)