A nationwide network outage in the United Airlines has caused flight delays and manual check-ins on Friday night.

On late Friday, United Airlines said in a message on its official Twitter account that “the computer outage was interrupting departures, airport processing and reservations. The airline was working to fix the issue.”

The statement did not mention the nationwide flight delays, while some travelers around the country posted unconfirmed accounts online claiming their flights had been delayed.

The disruption set off widespread delays at airports in San Francisco, Chicago and Washington and resulted in long lines of passengers at check-in counters.

Our technology team is working to resolve the issue as soon as possible. We apologize for the disruption being caused to travelers at affected airports and we are seeking to resume operations as quickly as possible, a company spokesman said, CNN reported.

Meanwhile, United Airlines may be fined nearly $600,000 after the company's drug testing process for flight crews failed to give them an equal chance of being selected for random drug and alcohol testing, the Federal Aviation Administration alleged Friday.

Thirteen United Airlines workers in key safety-sensitive positions were not properly tested for drugs, the FAA alleged.

Federal official proposed the fine on Friday, saying the company broke rules for random drug and alcohol testing.