Delta Air Lines warned Wednesday that flights may be delayed after a "technical issue" prevented passengers from booking, checking in and boarding.

"We apologize to our customers for the inconvenience they are experiencing as we work quickly to return our operations to normal," the airline said in a statement. "While we expect flight delays to extend into the evening at some of our busiest domestic hubs due to this issue and weather, we do not expect any technology-related cancellations."

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Delta did not immediately say what caused the issue or how many flights were delayed.

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The airline previously said a system-wide outage in August 2016 cost it $150 million in revenue. Outages have also hit American and United in recent years.

U.S. airlines suffered 34 computer outages between 2015 and 2017, and 85% of them caused flight delays and cancellations, according to a report by the Government Accountability Office that it published last week.

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