Spokeswoman Robin Urbanski said United Airlines, owned by UAL Corp., 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. CDT, when departures were able to resume after the computer functions returned, she said.

It wasn't immediately clear how many flights to and from Hawai'i were affected.

"Right now, our number one priority is to apologize for the inconvenience and work hard to resume full operations, which we expect to have done today," Ubanski said.

FAA spokesman Allen Kenitzer in Seattle confirmed that all United flights across its system had been grounded because of the computer problem.

By this afternoon, there were still delays on United flights across the country, and officials with the airline were working to tabulate how many flights had been affected. The airline has about 3,600 flights each day, Urbanski said.

At O'Hare International Airport, United's inbound and outbound flights were delayed between one and two hours and 29 United flights had been canceled, said spokeswoman Wendy Abrams.

No other airlines were affected.

Jocelyn Ashberg, from Capetown, South Africa, said she flew from Washington D.C. to Chicago today, only to sit on the tarmac at O'Hare for 1› hours after the plane landed.

"We rushed to get the plane in Washington because we were late coming in because we were delayed in Johannesburg," Ashberg said. "Then when we got to Chicago we just had to sit and sit and sit because nobody could tell us how long it was going to be ... we just had to sit like sardines in a can."

Michael McCarron, director of community affairs for the San Francisco International Airport, said about a dozen United flights were affected at the airport this morning before service resumed.

"Everything is back to normal right now," he said.

United is the airport's largest carrier.