PARIS--A strike by pilots at the French arm of Air France-KLM (AF.FR) that forced the airline to ground 60% of its planes has so far caused a net loss of between 30 million to 45 million euros ($39 million to $58 million), the chief executive said Thursday.

Air France will cancel about 60% of its flights on Thursday, for a fourth consecutive day, Alexandre de Juniac said in an interview with French radio station RMC.

The strike, which is due to end on Monday, has left thousands of travelers stranded at airports across France and around the world.

The estimated loss takes into account all costs associated with the strike and subtracts any savings made such as the fuel not burned, Mr. de Juniac said.

Pilots took industrial action to protest Air France's plans to expand its budget carrier Transavia while cutting costs at the legacy airline.

Write to Inti Landauro at inti.landauro@wsj.com

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