(Reuters) -Top U.S. carriers including Delta Air and United Airlines are restoring some operations on Friday after a technical issue related to an IT vendor forced multiple carriers to ground flights.

However, delays and cancellations were expected to persist throughout the day, as airlines try to fully recover from the impact of the outage that upended their flying schedules and affected thousands of passengers.

More than 1,400 flights were cancelled across the U.S., with nearly 4,000 delayed, as of 10:00 am ET on Friday, according to data tracker FlightAware.

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 hit banking, healthcare and a number of other sectors globally on Friday.

Delta and United said they were resuming some flights but expected additional delays and cancellations. The airlines also issued travel waivers for impacted passengers.

Peer American Airlines, which had earlier issued a ground stop notice, said it had safely re-established operations.

While American, Delta and United did not name the vendor, smaller carrier Frontier Airlines said that a “major Microsoft technical outage” hit its operations temporarily.

The issue stemmed from a defect found in a CrowdStrike content update for Microsoft Windows hosts, the cybersecurity firm’s CEO George Kurtz said on Friday.

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.

Parcel delivery firm FedEx said it faced substantial disruptions throughout its networks, while peer United Parcel Service also warned of potential delivery delays.

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.

(Reporting by Maria Ponnezhath, Shivani Tanna and Shivansh Tiwary; Additional reporting by Chandni Shah, Nathan Gomes, Surbhi Mishra, Angela Christy and David Shepardson; writing by Abinaya Vijayaraghavan and Shubham Kalia; Editing by Nivedita Bhattacharjee and Sriraj Kalluvila)

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