Issues have hit London's Gatwick and Heathrow airports, Charles de Gaulle in Paris and other locations including Washington DC, Melbourne and Singapore.
So far, airlines affected include at least Air France, Southwest, China Air, Qantas, Korean Air, Lufthansa and British Airways, according to CNN Money. "As a result of the incident, customers experienced disruption to certain services", a company spokesperson said in a statement emailed to NPR.
Such network problems are not at all unusual, but they're usually confined to specific airlines and airports.
Amadeus said that all systems were working normally by 9:30 am ET.
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The outage, which was caused by an unspecified "network problem", has now been resolved, but not before causing some delays at airports and problems for customers wanting to check bookings.
In a Facebook post, Changi Airport said the check-in process may take longer as manual boarding passes have to be issued.
Airlines around the world are reportedly experiencing issues with their check-in services following a systems failure.
A British Airways Airbus A320 just before landing at Heathrow.
Gatwick Airport said there had been "do delays". "We apologise for any inconvenience this may cause".
The problems were traced back to one software vendor, Amadeus, and its Amadeus Altea check-in software. "The action is ongoing with services gradually being restored".