Delta flights have gradually began taking off again after computer systems crashed, causing planes to be grounded and leaving passengers of one of the world's largest airlines stranded at airports around the globe.
The US carrier said the problems resulted from a power outage in Atlanta overnight and that customers should expect "large-scale" cancellations.
Delta said in a statement that a halt on departures had been lifted and some flights were resuming.
"Customers heading to the airport should expect delays and cancellations," it added.
The Flightradar24 website showed a flight from Phoenix to Atlanta had taken off, while three planes had departed from Amsterdam to US destinations.
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.
Delta operates 5000 departures a day and is a member of the SkyTeam alliance alongside airlines including Air France-KLM .
It also partners for transatlantic flights with Virgin Atlantic, which said its flights were operating normally but warned passengers to check tickets in case their flight was due to be operated by Delta as part of a code-share agreement.
The company's shares were down 1.1 percent at US$37.25 in pre-market trading.
In airports across the world, passengers stuck in check-in queues or on planes waiting to depart took to Twitter to share photos and frustration at the delays.
The glitch follows several high-profile computer problems faced by US airlines in the past year.
Budget carrier Southwest Airlines had to halt departures last month after a technical outage, while American Airlines had to suspend flights from three of its hubs last September after technical problems.
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.