Several airlines halted flights on Friday, while others warned of delays and services being unavailable as the impact of an unprecedented IT outage was felt globally.

Early on Friday, cybersecurity giant CrowdStrike experienced a major disruption linked to a tech update. Organizations including Microsoft were left scrambling to restore apps and services used by a huge number of firms.

Flight update and check-in monitors at airports around the world displayed the so-called “blue screen of death,” indicating a Microsoft system error. Belfast International Airport was using a whiteboard for staff to manually write out flight updates, Sky News’ Stephen Murphy reported.

“It seems that for the first time we are facing a real global blackout… The disruption affected not only individual users, but especially large institutions such as banks (including central banks), stock exchanges, airports, paralysing operations during the peak holiday season and causing chaos in many other sectors,” Grzegorz Drozdz, market analyst at Conotoxia, said in emailed comments.

Airlines across Europe, the Americas and Asia issued updates outlining the suspected extent of impact on their flight schedules and wider services.

Dutch airline KLM said Friday morning it had been forced to suspend “most” of its operations due to the outage, which it said had made it “impossible to handle flights.”

Its partner carrier Air France said at 10:36 a.m. U.K. time (5:46 a.m. ET) that operations were “disrupted,” excluding flights already en route, and that it would share further updates shortly.

Flights operated by Germany’s Lufthansa faced minimal disruption to and from Berlin, a spokesperson said; however, low-cost German airline Eurowings, part of the same group, said delays and cancellations were expected throughout the day due to issues with check-in and boarding.

Swiss air navigation service provider Skyguide said it had reduced the capacity of Swiss transit traffic by 30% as a precautionary measure after it was affected by the disruption.

In the U.S., the Federal Aviation Administration had a ground stop in place for American, Delta and United on Friday morning.

“The FAA is closely monitoring a technical issue impacting IT systems at U.S. airlines,” the Federal Aviation Administration said in a statement. “Several airlines have requested FAA assistance with ground stops for their fleets until the issue is resolved.”

American said that as of 5 a.m. ET it had been able to “safely re-establish our operation.” Delta said its flights were on pause and that affected customers would be contacted via the Fly Delta app and text message.

U.K. carriers British Airways and Virgin Atlantic both said some flight disruption was expected on Friday.

According to aviation analytics firm Cirium, Friday July 19 is set to be the busiest day of flights of the year, with the highest number of daily departures scheduled — 3,214 — since October 2019.

As of 11:45 a.m. in London, 1,396 flights had been canceled globally, a figure likely to change through the day, Cirium said.

London airports Gatwick and Heathrow both said they were using back-up systems to continue operations but said delays were expected. Gatwick said the issues spanned “some airlines’ check-in systems, baggage and security, including eGates.”

Singapore’s Changi Airport said self-check in systems were not working and check in processes were being managed manually. Singapore Airlines said its flights were currently operating as scheduled.

Cathay Pacific said self-service check-in facilities at Hong Kong International Airport had resumed following a technical outage.

“Facial recognition at bag drop facilities at Hong Kong International Airport remain unavailable. Please allow additional time for document checks if you are travelling with checked baggage,” the airline said.

— Leslie Josephs and Carlo Angerer contributed reporting.