American Airlines grounded all its flights across the U.S. on Tuesday after an unidentified computer problem hit its reservation system.
"We are now in a system-wide ground delay until 4:00pm CT as we work to resolve this issue. We apologize for any inconvenience," the airline said in a message on Twitter.
It had earlier confirmed the problem was with a reservation system, but more details were not immediately available.
The airline first posted that its system was offline shortly after 11 a.m. Central Time (16:00 GMT).
Martyn Williams covers mobile telecoms, Silicon Valley and general technology breaking news for The IDG News Service. Follow Martyn on Twitter at @martyn_williams. Martyn's e-mail address is firstname.lastname@example.org