The outage occurred between 8 a.m. and 10 a.m. Central Time, causing delays across the country. In total, about 268 domestic and international flights were delayed, United said. The average delay time was an hour and a half.

"We continue to work hard to resume operations by tomorrow morning and kindly ask for our customers' patience," the airline carrier said in a statement.

United does not know what caused the outage, but said that it "affected the systems that United uses to dispatch flights for departure."

The glitch delayed five United flights at Las Vegas McCarran International Airport, said Richard Viera, general manager of United operations there.

"We had a system outage and we worked with local people from United along with people from headquarters," said Viera.

The outage disrupted operations for about one-and-a-half hours but no flights were cancelled and Viera had no complaints from Las Vegas passengers. "It went pretty well," he said. By late afternoon, United flights were moving on time in Las Vegas.

This is the second time this month that a computer malfunction has caused significant delays in the U.S. On June 8, one of the two systems used by the U.S. Federal Aviation Administration (FAA) to manage flight plans failed, causing flight delays and cancellations across the country.