“Due to a technology issue, we are holding United mainline flights at their departure airports. We expect additional flight delays this evening as we work through this issue. Safety is our top priority, and we’ll work with our customers to get them to their destinations."

United Airlines said it had grounded its flights at U.S. airports on Wednesday due to a technology issue, and warned of additional flight delays in the evening.

All flights arriving at O’Hare Airport were halted, as well as flights heading to other airports in the U.S., including Denver and Houston.