American Airlines said its reservation system had an outage on Tuesday, leading the carrier to halt at least 200 flights.
"American's reservation and booking tool, Sabre is offline," American Airlines spokeswoman Mary Frances Fagan said in an email to Reuters.
"We're working to resolve the issue as quickly as we can. We apologize to our customers for any inconvenience."
FlightAware, a flight tracking service, said flights on American had been halted until 2 p.m. EDT, while regional flights on American Eagle at Dallas/Fort Worth, LaGuardia and Chicago O'Hare airports had been grounded until 2:30 p.m. EDT Tuesday.
"This directly impacts about 100 mainline flight and 100 Eagle flights, and will indirectly impact many more flights later in the day due to schedule disruptions," FlightAware's Mark Duell said in an emailed statement.
American, the AMR Corp unit that plans to merge with rival US Airways Group to form the world's biggest air carrier, operates more than 3,500 daily flights worldwide.