Delta Air Lines says it has suffered a computer outage throughout its system, and is warning of "large-scale" cancellations after passengers were unable to check in and departures were grounded globally. The No. 2 U.S. carrier said in a statement Monday that it had "experienced a computer outage that has impacted flights scheduled for this morning. Flights awaiting departure are currently delayed. Flights en route are operating normally." A power outage in Atlanta at about 2.30 a.m. local time is said to be the cause of computer outage. CNN reports: "Large-scale cancellations are expected today," Delta said. While flights already in the air were operating normally, just about all flights yet to take off were grounded. The number of flights and passengers affected by the problem was not immediately available. But Delta, on average, operates about 15,000 daily flights, carrying an average of 550,000 daily passengers during the summer. Getting information on the status of flights was particularly frustrating for passengers. "We are aware that flight status systems, including airport screens, are incorrectly showing flights on time," said the airline. "We apologize to customers who are affected by this issue, and our teams are working to resolve the problem as quickly as possible."