"This morning we experienced intermittent issues with our passenger service computer system, causing some flight delays. With the system now restored, we are working to get our customers to their destinations as soon as possible," the airline said in an emailed statement.
"We apologize for any inconvenience," the statement said.
The problem appeared to be nationwide. Denver airport officials confirmed the computer problems and delays around 7:45 a.m., according to Colorado news station Fox 31.
Chicago Department of Aviation spokeswoman Karen Pride said the airport is "aware that United is having some computer issues," but did not provide further information.
United customers at Dulles, LAX, and O'Hare complained of the computer issues on Twitter.
"Well, this is grand. All @united computers and systems down," @leeleechicago tweeted from O'Hare about 9 a.m.
Washington Redskins quarterback Kirk Cousins tweeted, "Haha-now all of @united computers are acting up at GR airport so we can't print our boarding passes to get thru security."
© Copyright 2014 Sun-Times Media, LLC