American Airlines says its systems have been fully restored after an outage grounded flights nationwide, but continued delays and cancellations are expected.
Flights across the U.S. were halted Tuesday when the main reservations system went down. Thousands of passengers were stranded at airports and on airplanes.
In a statement, the company said that if passengers must travel Tuesday and are already at the airport, they can rebook on another airline. If not yet at the airport, customers can rebook through American Airlines reservations or through another carrier and American will honor the fare difference.
"If a customer chooses not to travel today, there will be no charge for reservation changes or a full refund will be provided," the statement said.
The airline noted that it sees "no evidence that today’s technical outage is related to the tragic events in Boston."
"We will continue to provide more information throughout the day as we make progress in resolving this issue," the statement said. "We apologize to our customers and our people for this inconvenience."
Earlier, a spokesman for the Federal Aviation Administration said American asked to halt its flights until 5 p.m. EDT.
Flights into American's five biggest cities -- New York, Dallas, Los Angeles, Chicago and Miami -- are stopped until 5:30 p.m. EDT.
"Any American plane sitting on the ground anywhere in the U.S." won't be taking off, said FAA spokesman Lynn Lunsford.
Flight-tracking service FlightAware estimated that about 900 flights would be delayed if American resumed flying at 5 p.m., and the cascading effect would cause another 800 delays Tuesday evening because planes and crews wouldn't be where they were needed.
Customers couldn't make reservations or rebook if their flight was cancelled. Passengers described long airport lines and frustration at the lack of information from airline employees.
"Tensions are high. A lot of people are getting mad. I've seen several yelling at the American agents," said Julie Burch, a business-meeting speaker who was stuck at Dallas-Fort Worth International Airport waiting for a flight to Denver. "Nobody can tell us anything."
Terry Anzur, a TV news consultant from Los Angeles who was also stranded at DFW, said American Airlines gate employees were doing everything the old-fashioned, manual way because their computers were useless.
"No one at the counter can do anything. They can't check people in," Anzur said. "The airline is at a dead halt."
American's problems on Tuesday were reminiscent of what United Airlines passengers endured on several days last year. United experienced computer glitches after combining its reservations system with that of Continental after the two airlines merged. On one day in August, 580 United flights were delayed and its website was shut down for two hours. Another outage in November delayed 636 flights.
As with the United meltdowns last year, American Airlines passengers used social media to flood the airline with complaints.
"We are working to resolve this issue as quickly as we can, and we apologize to our customers for this inconvenience," said Andrea Huguely, a spokeswoman for the airline.
Huguely said all flights would be held on the ground until Tuesday afternoon, when she promised the airline would provide an update.
The airline was offering free reservations changes and refunds for stranded passengers. But Huguely said American couldn't process those changes until the reservations system was fixed.
The Associated Press contributed to this report.