© Rick Bowmer / AP
Stranded United Airlines passengers wait at the ticket counter at Denver International Airport on Friday.
Flights grounded or delayed at many large airports; airline working on problem.

Thousands of travelers were stranded at airports around the United States Friday night after a computer failure of United Airlines' reservation system.

United spokesman Charles Hobart confirmed the outage and said the airline apologized to customers.

"At approximately 7:15 pm CT tonight, United Airlines experienced a computer outage interrupting the airline's flight departures, airport processing and reservations systems," the airline said in statement. "Our technology team is working to resolve the issue as soon as possible."

Long lines of passengers formed at check-in counters at Chicago's O'Hare International and Denver International airports, two of United's largest hubs.

Passengers in Chicago told NBC station WMAQ that they were frustrated they were kept in line waiting for about two hours before any announcements came from the airline.

"I was only going home for 38 hours, so it's kind of cutting into that," said Sean Doyle, who was trying to board a 10:15 p.m. flight from Chicago to Denver.

"I'm a little cheesed, and I've been spending the entire time looking up who the new CEO of United Airlines is so I can compose a vitriol-filled note to him," said Kasey Madden, who was trying to fly to Minneapolis.

WMAQ later tweeted that passengers had been told that there would be no flights out of Chicago on Friday night.

At Los Angeles International Airport, United Airlines operations said there was a "nationwide computer malfunction." Employees were checking in passengers manually, said LAX spokeswoman Nancy Castles.

An NBC News producer at LAX reported that passengers were unable to check in unless they'd printed out their boarding pass prior to traveling to the airport.

The New York Times said passengers were reporting a chaotic scene at the United terminal at San Francisco International Airport and delays at Washington's Dulles International Airport.

Lines of frustrated fliers also were seen at Portland International Airport and at Seattle-Tacoma International Airport.

United is the second airline to have its operations disrupted by computer problems this month. Last week, a power outage grounded or delayed US Airways flights nationwide. Passengers couldn't make reservations, check in online, change their flights or check flight status.

NBC correspondent Jay Blackman, NBC producer Norma Rubio and NBC stations WMAQ in Chicago, KING in Seattle and KGW in Portland contributed to this report, which includes information from The Associated Press.