Frankfurt - Lufthansa pilots could extend the two-day strike that started at 06:00 GMT on Tuesday, German pilots' union Vereinigung said, to step up pressure on management in their long-running dispute over cost cuts, retirement benefits and pay.
"We cannot rule out further strikes this week," union spokesman Markus Wahl said on Tuesday. "Strikes are possible in the following weeks as well."
Tuesday's strike affects long-haul passenger and cargo flights out of Germany from 06:00 GMT to 21:59 GMT, with a further 24-hour walkout on Wednesday targeting short-haul Lufthansa and Germanwings flights.
Lufthansa has cancelled 84 of about 170 long-haul flights planned for Tuesday from Frankfurt, Munich and Duesseldorf because of the strike. It expects several hundred to be cancelled on Wednesday and will publish a list of cancellations on Tuesday.
The latest action is the 13th strike in 18 months at Germany's largest airline in a row that initially started over retirement benefits and has since escalated to encompass Lufthansa's plans to expand low-cost operations under the Eurowings brand. Strikes have cost Lufthansa about €100m so far this year.
Lufthansa, which is trying to cut costs to better compete with budget rivals in Europe, has been able to keep some long-haul flights and its cargo flights running on Tuesday thanks to pilots offering to work during the strike.
Relations between management and Vereinigung Cockpit soured last week after talks broke down. The pilots have offered concessions, including a proposed increase in the average retirement age to 60 and a commitment to look at ways to reduce costs to a level comparable with easyJet. However, it also wants Lufthansa to stop moving jobs out of Germany as it seeks to expand low-cost operations.
Lufthansa made some concessions over the weekend in an effort to persuade the pilots to return to the negotiating table, but they were dismissed by the union as lacking in substance.