UPDATE: United has lifted the ground stop.
All of United Airlines' domestic flights were grounded Sunday because of a computer outage, NBC News reported.
United later confirmed the news in a tweet which said: A ground stop is in place for domestic flights due to an IT issue. We're working on a resolution. We apologize for the inconvenience.
NBC said international flights weren't affected by the ground stop.
Fliers are urged to contact the airline for further updates.
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