NEW YORK (AP) - Passengers in several cities say a massive computer outage has stranded United Airlines passengers at airports across the country, but officials say systems have resumed normal operation.

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Thursday's outage was at least the third major computer outage for the Chicago-based airline since June.

In a series of tweets after the system failure, United Airlines officials said that they're aware of the problem and are working to fix it.

We're aware of a computer issue affecting some of our flights. We're working to resolve it. We'll update as we have more details.

— United (@united) November 15, 2012

However, in a tweet sent at about 10:52 a.m., the airline said that their system was "up" and that affected flights were resuming operations.

Passengers are being told by pilots and airport agents that computers are down and they don't know when the system will come back, passengers told The Associated Press. Some fliers have been waiting nearly 2 hours to depart.

A spokesman for United Continental Holdings Inc. did not immediately respond to a request for comment.