Major airlines have restored some operations after grounding flights across the US, citing communication issues caused by the global IT outage.
The outage, which has also affected banks and emergency services around the world, has been caused by a faulty software update from cybersecurity firm Crowdstrike.
Delta Airlines, which is the world's second-biggest carrier by passenger numbers, had paused all of its flights but later said some had resumed.
American Airlines also grounded its flights earlier but has resumed service.
Crowdstrike has apologised and said that the issue has been identified, isolated, and a fix has been deployed.
It is not currently known when the issue will be resolved.
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