Delta Air Lines said "limited departures" were resuming after a power outage that grounded flights worldwide just as summer holiday season got into full swing.
The airline said passengers should continue to expect delays and cancellations, and advised customers to check the status of their flights before they travel.
It added that large-scale cancellations were expected after the power cut, which caused flight status systems including airport screens to show the wrong status.
In a statement today, it added that the outages were largely affecting outbound flights.
"Our teams are working to resolve the problem as quickly as possible," it said.
In the UK, the airline serves Heathrow and Gatwick airports in London, and Edinburgh airport in Scotland. Departure boards at Heathrow appeared to show Delta flights were leaving, although some were delayed.
In the US, FlightRadar 24 showed the first Delta flight departing from Phoenix at 1.36pm UK time.