Pilots at Lufthansa's Eurowings began a three-day strike on Monday over working hours, their union said, confirming action that could affect thousands of the budget airline's passengers.
"It's begun. And so far there is no new offer," sais a spokesperson for the Vereinigung Cockpit (VC) pilots' union.
The union wants the carrier to ease pilot workloads through measures such as increased rest periods, but Eurowings warned over the weekend that the demands put jobs at risk.
The airline had said it would cancel nearly 170 of about 400 scheduled flights at airports in Duesseldorf, Cologne, Hamburg and Stuttgart on Monday, affecting roughly 20,000 travellers.
It expects to cancel a similar number of flights on Tuesday and Wednesday. The walkout is due to end at 11.59 pm local time (2159 GMT) on Wednesday.