Lufthansa has cancelled at least 300 flights as cabin staff staged another day of industrial action at three of Germany's biggest airports.
Strike action called by the UFO union was under way at Frankfurt, Munich and Berlin airports.
Lufthansa cancelled short and medium-haul flights but also services to and from Los Angeles, Houston, Chicago, Beijing and Mexico City.
UFO and Lufthansa are arguing over pay and working conditions.
The dispute appeared to escalate after UFO chief Nicoley Baublies threatened industrial action by the Lufthansa staff across Germany if the airline failed to compromise.
Lufthansa responded by saying that it was considering taking legal action against the union.
The strikes could cost between 5m and 10m euros (£3.9m-£7.9m) a day, but the figure could jump to as high as 50m euros if strikes become full-blown and all flights were cancelled, said Equinet analyst Jochen Rothenbacher.
Lufthansa spokesman Klaus Walther said: "It is difficult for the company to cushion the impact. We cannot just get new flight attendants, and the personnel buffer is limited."
The cancellations affect about one-sixth of the airline's daily number of services.
Lufthansa shares were down 1.19% in afternoon trade.
Workers held a day-long strike last Friday. The UFO union is seeking a pay increase of 5% for cabin staff for 15 months starting from January this year. The union said there had been no pay increases for three years.
It also wants guarantees that Lufthansa will not outsource jobs and use more temporary workers, as it has done in Berlin.
Lufthansa's latest offer was for a pay increase of about 3.5%.
The airline is undergoing a 1.5bn euro (£1.19bn) cost-cutting programme to offset rising fuel costs and tackle competition from low-cost and Gulf carriers.
It posted a 24% drop in second-quarter earnings to 229m euros from a year earlier. In May, Lufthansa announced it would axe 3,500 office jobs.
Lufthansa has about 19,000 cabin crew and the UFO union represents about two-thirds of them.