American Airlines said it had fixed a computer problem that prompted the grounding of its aircraft Tuesday, with flight resuming but schedules still in disarray.
Earlier in the day, American had grounded all its aircraft due to problems with its reservation system that started mid-morning and had caused "intermittent outages."
"As of 3:30 pm CDT (1930 GMT), our systems have been fully restored, however we expect continued flight delays and cancellations throughout the remainder of the day," the airline said.
"We will continue to provide more information throughout the day as we make progress in resolving this issue. We apologize to our customers and our people for this inconvenience."
An American spokesman said that 670 flights had been canceled.
The airline emphasized that the computer problems appeared to have no link to Monday's deadly bomb attacks at the Boston Marathon that killed three people and more than 170 injured.
"It is important to note we see no evidence that today's technical outage is related to the tragic events in Boston," it said.
American announced several options for customers booked to travel Tuesday.
For customers who needed to travel that day and were already at the airport, they can rebook on another airline with their electronic American Airlines ticket.
If customers must travel and are not yet at the airport, they can rebook through American Airlines reservations or through another carrier and American will honor the fare difference, the airline said.
Customers who choose not to travel Tuesday can make reservation changes at no charge or receive a full refund.
American, whose parent AMR Corporation is struggling to emerge from bankruptcy, plans to merge with US Airways to create the largest US airline.
The deal won US bankruptcy court approval in late March. Following other approvals required, the merger is expected to be completed by the third quarter of 2013.
The combined airline, to keep the American Airlines name, is to be led by current US Airways chief executive Doug Parker.