Southwest Airlines said there had been widespread intermittent issues concerning its flights at Ronald Reagan Washington National Airport, but no major disruption.
Travellers faced long waits at check-in desks after the outage from about 10.30am on Thursday.
Travellers are facing queues as mystery systems crash hits check-in desks at five global hubs. "Amadeus regrets any inconvenience caused to customers", the spokesperson said. We are working closely with our airlines to help resolve the issue as quickly as possible.
The problems were traced back to one software vendor, Amadeus, and its Amadeus Altea check-in software.
Several airlines at Terminals 2, 3 and 4 are affected.
Final update A network failure affecting the Amadeus online booking platform is continuing to delay check-ins for passengers around the world, although the firm won't disclose the cause.
The program, known as Altea and developed by a company called Amadeus, encountered problems as a result of what Amadeus called a "network issue".
Airports in Frankfurt, Paris, and Zurich reported of problems with their computer systems, which were quickly resolved.
Queues at the counters for Qantas, Lufthansa and Singapore Airlines - three of the airlines said to be affected - were moving normally. "That action is ongoing with services gradually being restored", a company spokesperson told FOX Business in a statement.