Major US airlines have grounded flights across the country, citing communication issues.
The grounding appears to be related to a global IT outage, which has affected banks, emergency services and airports around the world.
American Airlines, which is the world's biggest by passenger numbers, told the BBC that no flights were being allowed to take off, and that it was in contact with all flights that were currently in the air.
It blamed the problems on a "technical issue” with Crowdstrike, an antivirus software. Crowdstrike has not yet replied to a BBC request for comment.
Unconfirmed reports suggest the company issued a software update which has caused Windows devices to crash.
The Federal Aviation Administration (FAA) said in a notice to carriers that "all... flights regardless of destination" had been grounded, according to a report from the AFP news agency.
Delta and United airlines have also been affected by the tech outage, as well as a number of smaller carriers.
Late on Thursday, US airline Frontier said a "major Microsoft technical outage" had affected its operations.
Commenting on that development, US Secretary of Transportation Pete Buttigieg said his department was "monitoring technical issues” at Frontier that were “leading to cancellations and delays across their network."
Flights have also been grounded in other countries around the world.
Japan's Narita airport, around 60km (37miles) from Tokyo, says airlines JetStar, Jeju Air, Qantas, HK Express and Spring Japan are having issues with their systems.
And in India, Delhi airport says some services have been temporarily impacted.
Amsterdam's Schiphol airport is one of the latest to report delays caused by the IT outage.
"The outage has an impact on flights flying from and to Schiphol," a spokesperson said, adding it that it was not yet clear how many flights had been affected.
