Flights resumed at about 5 p.m. EDT but the airline said it expects "continued flight delays and cancellations throughout the remainder of the day."
The airline had canceled an estimated 200 flights by just after 4:30 p.m. and was advising travelers to call ahead to confirm the status of booked flights, flight-tracking website FlightAware.com said. Hundreds more flights have been delayed, and about 2,000 flights have been affected directly or indirectly, through a domino effect.
"This is a major outage for American and is the longest flight disruption as a result of a failure of an airline's back-end technology in recent history," FlightAware Chief Executive Officer Daniel Baker told USA Today. "It is likely to affect over 125,000 travelers today and tomorrow."
Baker said United Airlines had "a few similar outages" in 2011, some related to technology problems associated with the United/Continental merger, but none lasted as long as American's persisted Tuesday.
FlightAware said most of American's delays and cancellations Tuesday involve arrivals and departures at the airline's main hub at Dallas/Fort Worth. Miami, Chicago O'Hare, and New York LaGuardia also experienced problems.
"American's network system is experiencing intermittent outages," the company said in a statement announcing it had grounded all flights through 5 p.m. "At this time, we are holding all flights on the ground until later this afternoon, when we will provide another update."
The statement said the airline is "working to resolve this issue as quickly as we can, and we apologize to our customers for this inconvenience."
"If your travel plans are flexible, there will be no charge if you would like to change your reservation and we will provide full refunds if your travel plans are not flexible," the statement said. "However, we are unable to make changes to current travel plans until we have resolved this issue."
The airline had said earlier Tuesday it was having problems with its reservation and booking system, CNN reported.
American says it flies about 275,000 passengers a day and handles more than 239,000 reservations calls, CNN reported.
AMR Corp., the airline's parent company, filed for bankruptcy protection in 2011. Plans call for American to merge with US Airways, the Los Angeles Times said.