German airline Lufthansa cancelled about 800 of its flights on Monday after more than 4,000 of its pilots began a four-day walkout.

The airline, Europe’s biggest by sales, said that despite the strike organized by the Cockpit pilots union, it was maintaining many domestic flights and short-haul routes across Europe though many of its long-haul flights to the U.S., including New York and Denver, were cancelled.

Other flights to Africa, South America and Asia were still scheduled to operate.

“Usually we have 1,800 flights a day,” Deutsche Lufthansa AG said early Monday.

“For today, we foresee about 1,000 flights planned, but there may be more flights that could be cancelled during the day,” the airline warned. It offers some 160 long-haul flights.

Pilots for Lufthansa Cargo and Lufthansa’s its low-budget subsidiary, Germanwings, are also taking part in the strike.

Lufthansa, based in Cologne, owns or holds significant stakes in airlines including Swiss International Airlines, Austrian Airlines, JetBlue of the U.S. and Britain’s BMI. Those are not affected.

The pilots are seeking increased job security and want German labour conditions to apply to Lufthansa pilots hired abroad, in an effort to prevent their jobs from migrating to neighbouring countries with cheaper conditions.

The airline said it was trying to rebook travellers on partner airlines or trains. Travellers unable to be rescheduled are being reimbursed for their tickets, it said.

The airline, Germany’s largest, estimates the strike could cost it some euro25 million ($34 million) per day.

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