“It’s (an) all-hands-on-deck effort” at Delta to get customers back into the air, Chief Executive Ed Bastian said in a video posted on the airline’s Twitter account.

The problems 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.

According to website Flightradar24, some of the first flights to take off were from Amsterdam to the United States, while a flight from Phoenix to Atlanta was among the first to depart from a US airport.

Delta operates 5,000 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 cautioned that passengers should check tickets in case their flight was due to be operated by Delta as part of a code-share agreement.

Delta’s system failure did not affect any inbound or outbound flights from Hong Kong.

“Delta experienced a computer outage that has impacted flights scheduled for this morning,” said Delta, the world’s second largest airline measured by revenue passenger kilometres flown.

The outage was affecting flights globally, a London-based spokeswoman for the airline said.

Like many large airlines, Delta uses its proprietary computer system for its bookings and operations, and the fact that other airlines appeared unaffected by the outage also pointed to the company’s equipment, said independent industry analyst Robert Mann.

Critical computer systems have backups and are tested to ensure high reliability, he said. It was not clear why those systems had not worked to prevent Delta’s problems, he said.

“That suggest a communications component or network component could have failed,” he said.

Passengers stuck in check-in queues in airports across the world, or on board planes waiting to depart took to Twitter to share photos and frustration at the delays.

“1 hr.+ lines @HeathrowAirport for @Delta due to system outage,” tweeted user @MITJAKE with a picture of passengers waiting to check in.

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,” Resende 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.

“This is ridiculous,” said Nyasha Arthur, a 39-year-old AT&T employee who had to use a vacation day after being stranded at Newark Liberty International Airport.

“I don’t understand what is going on here. It’s just a mess,” she said as she stood in a long queue at Delta’s check-in counter.

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.

In March, a computer system malfunction forced Japan’s All Nippon Airways to cancel more than 100 domestic flights, affecting some 16,000 travellers.

And in mid-August 2015 a computer problem at a regional air traffic control centre delayed hundreds of flights at busy US east coast airports - including those in the Washington and New York areas - for several hours.

A rival US airline, United, suffered computer glitches in May and July 2015 that temporarily grounded hundreds of flights and backed up thousands of passengers.

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.

Additional reporting by Agence France-Presse