American Airlines said Tuesday it had grounded all its aircraft due to problems with its reservation system.
"American's network system is experiencing intermittent outages," AA said on its website.
"We are on a system-wide ground delay that will last until 4:00 pm CST (2100 GMT) as we work to resolve this issue as quickly as we can," the company said, apologizing to customers for "any inconvenience."
In a Twitter message, AA said: "The issue is w/ our ability to access our res system & not w/@SabreNews. We apologize to Sabre & customers for confusion.
In an earlier tweet, AA had said the Sabre reservation and booking tool was offline.
American, whose parent AMR Corporation is struggling to emerge from bankruptcy, plans to merge with US Airways to create the largest US airline.
It would also help lift AMR from bankruptcy reorganization, where it has been since November 2011.
The deal won US bankruptcy court approval in mid-February.
The combined airline, to keep the American Airlines name, is to be led by current US Airways chief executive Doug Parker.