All United Continental flights in the U.S. are currently grounded due to computer problems.
United said in a statement on Wednesday that it is working to resolve the problems, which are related to "network connectivity."
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It is the second time in two months that the Chicago carrier has been hit by major technical issues.
United Continental Holdings Inc. had a temporary halt to all takeoffs in the U.S. on June 2 because of what the airline said were computer automation issues.
Its stock declined slightly in premarket trading.