Delta Airlines flights resumed around 8:30 a.m. Monday after a computer system outage grounded planes for hours.
The airline warned passengers to expect continued delays.
“While inquiries are high and wait times are long, our customer service agents are doing everything they can to assist,” the company said in a statement.
The delays are affecting flights across the world.
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A power outage in Atlanta that started at 2:30 a.m. affected computer systems, the company said.
“Large-scale cancellations are expected” Monday, according to a Delta update at 7:30 a.m.
As of 10:30 a.m., 300 flights were canceled and 800 of about 6,000 flights were up and running.
The delays are frustrating travelers.
Among those affected by the situation is Stephen Smith, 32, of Baton Rouge, Louisiana. He has been stuck on the ground for about three hours at Tokyo's Narita Airport on a flight that was supposed to go to Shanghai.
Smith took solace in the fact the air conditioning on the plane was working and said it seemed everyone on board was fine.
"Waiting game at this point," he tweeted to The Associated Press.
“We apologize to customers who are affected by this issue, and our teams are working to resolve the problem as quickly as possible,” the company said in a statement.