About 4,000 pilots at German airline Lufthansa have gone on strike for four days in a dispute over job security.
The carrier has cancelled about 3,000 flights and has warned of delays both domestically and internationally.
The strike has disrupted thousands of passengers around the world, while the company tries to arrange alternative travel for them.
The pilots work at Lufthansa, Lufthansa Cargo and budget subsidiary, Germanwings.
Lufthansa's offer of negotiations with the pilots was not taken up on Sunday before the strike began at midnight (2300 GMT).
The airline normally offers about 1,800 flights daily - of which 160 are long-haul trips.
Lufthansa has said about two-thirds of flights will have to be cancelled during the strike.
Lufthansa says that affected passengers can rebook their ticket once to another Lufthansa flight free of charge, provided the original ticket was issued before 18 Feb and the new travel date is before 31 May
Anyone wishing to cancel their flight is entitled to a full refund
The airline is offering train journeys to domestic air travellers and attempting to rebook international passengers on other airlines
In general, customers are extremely unlikely to have any claim for extra compensation, because the cancellation is not "within the airline's control"
If booked as part of a package holiday, contact the agent or operator who will try to find alternative flights but, if that is not possible, will refund the cost of the whole package holiday
Anyone who tries to claim for holiday costs through their insurance, should contact their insurer for guidance
It was also reported that the company had asked a German court to halt the strike.
Lufthansa spokeswoman Claudia Lange told the Associated Press that an injunction had been filed in Frankfurt.
"This strike is disproportionate," she said. "We hope for a decision within the next 24 hours."
Before the strike began, German Transport Minister Peter Ramsauer warned that it would hurt the country's economy as well as Lufthansa's reputation.
The airline - one of the world's largest - fears the strike could cost it about 25m euros (£21.9m; $34m) per day.
The pilots are not due to return to work until midnight on Thursday.
The Cockpit union says the airline is increasingly relying on foreign pilots who fly for less pay.
It is concerned that the company could try to cut staff costs by shifting jobs to foreign subsidiaries such as Austrian Airlines or Lufthansa Italia, where wages are lower.
The starting salary for a first officer in a Lufthansa cockpit is 62,000 euros, and 115,000 euros for a captain, according to the company's recruiting website. Media reports say pilots' salaries can rise up to about 325,000 euros.
Cockpit has called for a 6.4% pay rise for pilots, more say in company decisions and commitments that pilots would keep their jobs when Lufthansa moves passengers to cheaper foreign affiliates.
But Andreas Bartels from the airline told the BBC the pilots' fear that their jobs would be outsourced was unfounded.
"That's what they fear but that's not reality. If you look to the reality, it's nothing like replacing or transferring jobs to other companies or other airlines [in] the Lufthansa airline system," he said.
He added that Lufthansa would not be dictated to by the unions.
"It's not about money. The unions made it very clear that they're willing to make a compromise when it comes to their claim on money, but it's about political influence on the company's strategy and that is something that we can't accept."
Lufthansa was offering train journeys to domestic air travellers, and attempting to rebook international passengers on other airlines.
Frankfurt and Munich airports will be the worst hit.
Hours before the strike began, Lufthansa made a last-ditch effort to resume talks.
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