Some 240 flights were canceled at Frankfurt airport Monday due to a strike by ground workers, German airport operator Fraport AG said. Around 190 of Fraport’s traffic-operation-center employees, apron controllers and apron supervision staff went on strike Thursday and Friday seeking better pay and conditions. Their union, GdF, said earlier Monday that the warning strike will continue until 11 p.m. ET Tuesday.

The strikes on Thursday and Friday cost Fraport a total of €3.5 million to €4 million in lost sales, the company said, adding it would lose €5 million a day if all air traffic was canceled. Silvia Quandt Research estimates the strike hurts the company’s operating earnings by €3 million to €5 million a day.Despite the flight-service disruptions, operations at the airport were running smoothly, Fraport spokesman Jürgen Harrer said, adding that replacement personnel had been brought in to alleviate the disruption.

A spokeswoman for Germany’s largest airline group, Deutsche Lufthansa AG—the carrier most affected by the industrial action—said it will need to cancel only 160 flights on Tuesday, fewer than previously planned. The cancellations affect domestic and intra-European flights, not intercontinental flights, she added. On Friday, Lufthansa said it had cancelled 232 flights. She declined to comment on the cost of the cancellations.

Thursday and Friday’s strikes resulted in the cancellation of around 30% of scheduled flights at Frankfurt airport.At present, Fraport is looking for temporary assistance from personnel at other international airports and has been training some staff in different roles. The company is prepared for further industrial action in the coming days, the Fraport spokesman said.On Sunday, Fraport said the group aimed to maintain 50% of scheduled flights Monday, and urged labor-union representatives to return to the negotiating table.

There was no contact between the union and the company over the weekend, Mr. Harrer said Monday, adding the union needs to compromise.Separately, union Chairman Markus Siebers said that while the strike was having an effect, workers were disappointed Fraport hasn’t relented yet. Until Fraport is prepared to compromise, the union has sufficient funds to continue the strike, he said. The union is prepared to return to the bargaining table, “but we can’t start from square one again, Mr. Siebers said.