BERLIN (AP) — Ground staff for Lufthansa walked off the job at five major German airports on Wednesday, causing the airline to cancel hundreds of flights.

The Ver.di union called on ground staff at Frankfurt and Munich, Lufthansa's two main hubs, as well as Berlin, Duesseldorf and Hamburg, to strike for 27 hours starting at 4 a.m. Wednesday.

Lufthansa said ahead of the strike that it expected to be able to operate around 10-20% of all planned flights. It said tickets could be rebooked free of charge, and tickets for German domestic flights could be converted to rail vouchers.

In Frankfurt, the company canceled 80-90% of the planned 600 departures and arrivals by Lufthansa itself and subsidiary Air Dolomiti ahead of the strike, German news agency dpa reported. More than 400 departures and arrivals at Munich Airport also were canceled.

All Lufthansa departures from Berlin and Hamburg were canceled, as were most domestic flights from Duesseldorf. Lufthansa anticipated that, in all, more than 100,000 people would have to change their travel plans.

Lufthansa unit Eurowings said it wasn't affected.

The union is seeking a 12.5% pay raise, or at least 500 euros ($539) more per month, in negotiations for nearly 25,000 employees, including check-in, aircraft handling, maintenance and freight staff. Hourslong or one-day “warning strikes” are a common tactic in German contract negotiations.

Coinciding contract negotiations have resulted in several recent walkouts in the rail, air and local transport sectors, making for a frustrating few weeks for travelers and commuters in Germany.