Microsoft said early on Friday that its cloud services outage in the Central US region was resolved after it led to the grounding and cancellation of several flights.

Low-cost carriers Frontier Airlines, a unit of Frontier Group Holdings, Allegiant and SunCountry, had 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.

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”.

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.

“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 canceled 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.