A global IT outage has swept major services worldwide, disrupting public transport links, delaying international flights and restricting operations for health care facilities and businesses.

Here’s what you need to know:

What is behind the outage? The outage appears to stem at least partly from a software update issued by Crowdstrike, on Microsoft Windows systems. The US cybersecurity firm told customers early Friday that engineers were addressing the problem, according to an advisory viewed by CNN. The issue is specific to Falcon, which is designed to protect files saved in the cloud.

How long will the outage last? Microsoft said the “underlying cause” of the outage “has been fixed,” adding that residual impact is still affecting some services. “We’re conducting additional mitigations to provide relief,” the tech company said in a post on X. Some services have been restored, but others are still grappling with the disruptions.

Who has been affected? Global banks, medical services and critical infrastructure have been impacted by the outage. Medical providers including the UK’s National Health Service, and one of Europe’s largest health facilities in northern Germany, have been stifled by IT delays. Meanwhile, banks, broadcasting outlets and supermarkets in Australia, New Zealand and the UK were affected, including ABC News and Sky News.

Airlines worst hit: Scenes emerged of flustered travelers filling the corridors of major airports on Friday, as US and other international carriers in Europe, Asia and the Middle East had to delay and cancel flights. There are around 110,000 scheduled commercial flights today worldwide, according to preliminary data from Cirium, an aviation analytics company, shared with CNN. As of 6 a.m. ET, there were 1,390 canceled flights globally. That figure is growing.

International response: The White House says it is “looking into” the disruption caused by Friday morning’s cyber outages. “We’re aware of the incident and are looking into the issue and impacts,” a White House National Security Council spokesperson told CNN. Meanwhile, the UK government held an emergency meeting to discuss the outage, Downing Street said, according to the UK’s PA Media news agency.