Thousands of international travelers have been left stranded as a strike by Lufthansa pilots was extended to long-haul flights.

Germany's largest airline had to cancel 84 of 153 of its long-haul flights on Thursday, affecting 18,000 passengers, the dpa news agency reported.

Already, some 80,000 passengers were affected in the first day of the strike Wednesday when some 750 short- and medium-haul flights were canceled. Similar disruptions were expected for Friday.

The Vereinigung Cockpit union has hit various Lufthansa units with around a dozen short-term walkouts over recent months.

The dispute centers on the airline's plans to cut transition payments for pilots wanting to retire early, which the union wants to see maintained.

The union wants to cut those payments, citing increasingly tough competition.