Tens of thousands of passengers were stranded on Monday after Delta Air Lines flights were grounded around the globe due to a system outage.
Flights gradually began taking off again after a power outage hit Delta's computer systems, grounding planes and stranding passengers of one of the world's largest carriers.
The US airline said the power outage began at about 2:30 am EDT (0630 GMT) in Atlanta and that customers should expect "large-scale" cancellations.
Delta said in an update at 1240 GMT that a halt on departures had been lifted, and some flights were resuming, although customers should expect delays and cancellations.
The company said via Twitter that it had cancelled about 300 flights due to the power outage.
The problems also meant flight information was not showing correctly on Delta's website or on airport information boards, and this could also take time to resolve, the carrier said in the latest update.
"Customers heading to the airport should expect delays and cancellations," Delta warned in a statement. "While inquiries are high and wait times are long, our customer service agents are doing everything they can to assist."
The airline also advised fliers there may be "some lag time in the display of accurate flight status at delta.com, the Fly Delta App and from Delta representatives on the phone and in airport".
As for the cause of the snafu, Delta pointed to an overnight power outage in its hometown of Atlanta that it said "impacted Delta computer systems and operations worldwide, resulting in flight delays".
However, Georgia Power disputed that assessment.
''We believe that Delta experienced an equipment failure overnight that caused a power outage,'' Craig Bell, a spokesman for Georgia Power, told USA TODAY. ''Our crews have been out there all morning closely with Delta and their team as they repair the affected equipment. We don't believe it was a Georgia Power issue.''
Regardless of the cause, the glitch knocked Delta's automated check-in kiosks offline, forcing agents at ticket counters to manually check-in some passengers.
"Large-scale cancellations are expected today," Delta said in an earlier statement from 7:30 am ET.
Fliers took to social media with images of growing lines at a number of locations served by Delta.
''The timing of this problem is particularly bad because Monday morning is one of the busiest times for both airlines and travelers as business travelers begin their work week,'' Daniel Baker, CEO of flight-tracking service FlightAware, said on Monday.
In response to the disruptions, Delta is waiving change fees for customers ticketed to travel Monday. Customers booked on Monday's flights can make one change to their itinerary without paying the standard change fee, which ranges from $200 domestically to as much as $500 on some international itineraries. Delta says rebooked travel must be for flights no later than Friday.
The outage will hurt Delta's reputation for punctuality and lack of cancellations.
Delta had the fewest cancellations in May at 39, which counted as a statistical zero, according to the most recent flight statistics from the Transportation Department. For comparison, the least reliable airlines cancel more than 1 per cent of their flights.
While discussing second-quarter earnings, Delta CEO Ed Bastian said the mainline airline had 77 days during April, May and June without any cancellations, in what he called the industry's best operation. The airline had 23 days without any cancellations in the system of nearly 6,000 daily flights, he said.
Industry consultants say airlines face an increasing risk from computer disruptions as they automate more of their operations, distribute boarding passes on smartphones and fit their planes with Wi-Fi.
The issue comes less than three weeks after Southwest Airlines blamed a router failure for starting a cascading technology glitch that knocked reservations systems offline and took several days to fully resolve. It wasn't until 25 July that Southwest said it was back to a near-normal operation, something that came after more than 2,300 cancellations - about 12 per cent of its schedule over the period. Flight-tracking service estimated nearly 8,000 flights arrived late.
Delta said it serves nearly 180 million customers annually, employs more than 80,000 people around the world and operates more than 800 aircraft.