Image copyright Reuters Image caption Delta passengers across the US were severely affected by the power cut

Thousands of air passengers around the world have been left stranded after a power cut forced the US airline Delta to suspend flights.

The incident caused delays across the US and in Japan, Italy and the UK.

Airport check-in systems, passenger advisory screens, the airline's website and smartphone apps were affected by the systems failure on Monday.

Flights resumed six hours later but Delta warned of continuing delays as a backlog of passengers was cleared.

The airline suspended or cancelled dozens of departures early on Monday, with airport agents forced to write out boarding passes by hand.

By Monday evening, Delta said more than 740 flights had been cancelled but its computer systems were working again.

Delta offered passengers refunds and $200 (£153) vouchers while CEO Ed Bastian apologised for the inconvenience in a video message.

"The Delta team is working very hard to restore [services] and get the system back [operating] as quickly as possible," he said.

The overnight power failure took place in Atlanta, near Delta's headquarters, the company said, causing computer systems to crash.

Image copyright EPA Image caption Delta serves about 180 million customers a year, employing more than 80,000 people

Image copyright AP Image caption Some flights from Atlanta have now resumed

"Our systems are down everywhere," the company at one point told its customers on Twitter.

Passengers around the world complained of long check-in queues, with many sleeping on the floor.

"This is ridiculous," one of hundreds of passengers stuck at Newark Liberty International airport in New Jersey told Reuters news agency.

"I don't understand what is going on here. It's just a mess," she said while waiting at Delta's check-in counter.

Another passenger complained of "utter confusion across the board" on social media while another said the airline had not provided enough staff at check-in counters to explain what was going on.

Many travellers posted their unhappiness on social media, making #Delta a top trending topic.

After the problem was resolved, the airline warned that many passengers still faced delays and cancellations because of the knock-on effect of the earlier disruption.

Delta ranks third in the world in terms of passengers carried, according to industry body the IATA.

The airline has advised customers to check the status of their flight before heading to the airport.

Image copyright EPA Image caption Passengers complained of long queues at check-in desks

Image copyright AP Image caption Flights across the US have slowly been returning to normal

The incident will damage Delta's reputation for punctuality and lack of cancellations, USA Today commented.

While Delta has one of the best reliability records in the industry, the Wall Street Journal reported, its "relative outperformance" has left stranded passengers with fewer options when it comes to allowing them to rebook at discounted fares.

One passenger told the BBC he was waiting with "several hundred" fellow stranded passengers at San Francisco airport, after being asked to leave a plane he had boarded.

"We were ordered off the plane after approximately an hour or more," said Dick Ginkowski, who said that tempers were starting to get strained.

Delta has been responding to unhappy customers on Twitter and has issued guidelines for affected passengers.

Image copyright Twitter Image caption A typical exchange between Delta and a social media user

The airline serves about 180 million customers a year, employing over 80,000 people, its website says.

It is the latest carrier to suffer computer problems, with Southwest Airlines last month forced to cancel more than 2,000 flights after an outage prevented travellers checking in.