A computer malfunction at United Airlines in Chicago stopped all United departures systemwide for two hours Wednesday.
Spokeswoman Robin Urbanski said the airline did not yet know what caused the problem, which she described as a computer outage.
The outage lasted from 8 a.m. to 10 a.m. local time (1300 GMT to 1500 GMT), when departures were able to resume, she said.
"We were experiencing a computer outage and now our computers are back up," she said. "Right now we're working hard to resume our operations."
U.S. Federal Aviation Administration spokesman Allen Kenitzer in Seattle confirmed that all United flights across its system had been grounded because of the computer problem.
At O'Hare International Airport, United's inbound and outbound flights were delayed between one and two hours, and no other airlines were affected, airport spokeswoman Wendy Abrams said.
"We just had to sit like sardines in a can," said one passenger, Jocelyn Ashberg of South Africa, who said her flight sat on the tarmac at O'Hare for 1 1/2 hours after the plane landed.
Courtney Prebich, a spokeswoman for the Metropolitan Washington Airports Authority, said the delay of United flights at Washington Dulles International Airport did not affect departures on any other airlines.
Michael McCarron, director of community affairs for the San Francisco International Airport, said about a dozen United flights had been affected at the airport, and most delays averaged 20 to 30 minutes. United is the airport's largest carrier.
"I expect it to decrease as the day goes on," he said. "And by noon, (it will) be pretty much back on track."
United's hubs are O'Hare, Dulles and airports in Denver, Los Angeles and San Francisco.