American Airlines is having some computer trouble, and it seems to be pretty bad. The Federal Aviation Adminstration (FAA) is grounding all of the company's airplanes at the carrier's request. American Airlines estimates that planes will stay grounded until at least 5:00PM ET, as the carrier attempts to fix access to its online booking and reservation system. According to the FAA, the outage also affects regional American Eagle flights departing for Dallas Fort-Worth, LaGuardia and Chicago O'Hare airports, and could impact flights in general.

At that point, the problem will have lasted over five hours. American Airlines first tweeted the issue at 12:19PM ET, and some customers are complaining on Twitter that their planes have been stuck on the tarmac for multiple hours already. American Airlines says that it will offer refunds and update travel plans at no charge for affected customers, but won't be able to do so until it fixes the computer problem.

A statement on American Airlines' website reads: "American's network system is experiencing intermittent outages. At this time, we are in a system-wide ground delay that will last until 4:00pm CST as we work to resolve this issue as quickly as we can. We apologize to our customers for any inconvenience."

Initially, the carrier blamed booking and reservation provider Sabre for the outage, but a Sabre spokeswoman told Bloomberg that there were no problems with its system. American Airlines has since clarified that access to the booking system is the issue, and apologized to Sabre for the accusation.

The Dallas Morning News is following the situation live. According to a memo allegedly sent to American Eagle pilots, American is "experiencing a complete FOS outage." Pilots at some airports may be asked to move departing planes away from the gates to make room for more incoming flights, which could lead to further outgoing delays. Dallas Fort-Worth has reportedly provided large buses to help passengers off planes when gates aren't available.

FlightStats, which tracks flights around the world, estimates that American Airlines has canceled 389 flights as of around 4:30PM ET, and that American Eagle has canceled 291 flights, compared to only 16 and 13 (respectively) yesterday. American Airlines confirmed 670 total cancellations as of 5:15pm ET.

Update 4:23pm ET: An American Airlines spokesperson tells Bloomberg that computer systems are "back up and running" and that flights will resume "soon," according to this tweet.

Update: 4:37PM ET: American Airlines says "our systems have been fully restored," but expects "continued flight delays and cancellations throughout the remainder of the day." More details at the carrier's Facebook page.

Update: American Airlines provided The Dallas Morning News with the following statement:

Flights from our hubs and international flights having been re-started, and we will reposition aircraft and crew throughout the evening.

Despite the magnitude of today's disruption, we are pleased to report that we expect our operation to run normally with only a small number of flight cancellations expected tomorrow.

We will add additional flights, as needed, tomorrow to ensure our customers are able to continue their travels. Again, we sincerely apologize to our customers for this inconvenience and thank our team for their extraordinary efforts today.

Update 8:55pm ET: American Airlines chairman and CEO Tom Horton apologizes in this YouTube video: