WASHINGTON • Delta Air Lines scrapped hundreds of flights and delayed many more as it digested the aftermath of a computer meltdown on Monday.

Even after the US carrier lifted its initial flight-grounding order about six hours after the computer snafu struck, it warned of continued cancellations and delays, as tens of thousands of passengers around the world were left stranded.

Travellers crowded ticket counters, stretched out on the ground in airports and tried to grin and bear it. Ticket agents were reduced to checking people in manually, news reports said.

Nearly 250 flights were expected to be cancelled on Tuesday morning, the airline said as it continued recovery operations.

A power outage at Delta's Atlanta base about 2.30am local time on Monday interrupted computer operations, resulting in the cancellation of about 1,000 flights the same day, it said.

Delta has more than 15,000 flights a day, along with its alliance partners.

"While systems are improving and flights are resuming, delays and cancellations continue," the carrier said in a statement on Monday.

Teams were working "round the clock" to bring systems back online, Delta CEO Ed Bastian said in a video message from the airline's operations centre.

He added: "I apologise for the challenges this has created for you with your travel experience."

Mr Luciano Resende, 40, waiting at London's Heathrow Airport to fly to San Francisco, said it was slow going. "I guess it has been a long time since they used the manual process," he told NBC News.

At Los Angeles International Airport, people waiting for red-eye flights to the east coast dozed in a crowded waiting area, many of them wrapped in red blankets.

Monday is never fun, but this one was worse. "The timing of this problem is particularly bad because Monday morning is one of the busiest times for both airlines and travellers as business travellers begin their work week," said Mr Daniel Baker, FlightAware's CEO.

As compensation, Delta offered refunds to travellers whose flights were cancelled or significantly delayed. People on some routes were being allowed to make a one-time change to their travel plans.

A vast number of flight delays normally creates a cascading problem that affects airline traffic for days.

Computer outages halting flights are not uncommon.

In May, a glitch affecting Sweden's civil aviation authority radar site disrupted air traffic throughout that country and grounded flights to and from Stockholm for several hours.

The disarray at Delta marked the second time in less than a month that a system failure forced mass flight cancellations at a large US carrier.

Southwest Airlines, the biggest US discount airline, said a disruption on July 20 would cost it "tens of millions" of dollars after more than 2,300 flights were cancelled.

AGENCE FRANCE-PRESSE, BLOOMBERG