Passengers remain grounded as Lufthansa Airlines cancelled 50 flights ahead of a planned 24-hour walkout by cabin crew at six major airports on Friday.
Thursday's strike, the latest in a round that has left thousands stranded, affected mainly domestic and European services, though some intercontinental flights, including New York-Frankfurt and Hong Kong-Munich were also hit.
On Tuesday, the carrier had cancelled over 200 flights in and out Germany.
The series of cancellations, affecting the German flag carrier's Frankfurt hub, as well as several other German cities, came as a union chief threatened to hit every airport in Germany with a 24-hour strike on Friday.
Late Wednesday, the cabin staff's labour union, Independent Flight Attendants' Organisation, UFO, said its members will stage a 24-hour stoppage on Friday at the airports of Frankfurt, Berlin, Hamburg, Munich, Duesseldorf and Stuttgart in an escalation of their ongoing pay and conditions dispute.
Lufthansa has therefore said it will cancel 1,200 flights, or two-thirds of its total 1,800 flights on Friday.
The airline already faces headwinds because of rising fuel prices and fierce competition.
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.
"We're prepared to go to mediation on the issue of pay hikes. But negotiations cannot include the use of temporary staff," a Lufthansa spokesman told the AFP news agency.
Lufthansa and UFO traded verbal blows earlier in the week, with each accusing the other of arrogance.
Lufthansa had shown "no sign of bending", Dirk Vogelsang, who is leading the negotiations, told the AFP news agency, adding that "at the moment, it looks very, very difficult" with the attitude hardening on both sides.
UFO is seeking a five per cent pay increase for cabin staff backdated to January after three years of wage freezes. It is also opposed to the use of temporary cabin staff on Lufthansa aircraft.
Lufthansa spokesman Klaus Walther said the airline had offered a salary hike of about 3.5 per cent as well as an offer to stop fixed-term contracts and halt the use of temporary staff.
"UFO has not given us a contribution towards increasing competitiveness," such as working an extra two hours a month, he complained, calling for both parties to return to the negotiating table.
He said the airline would do everything possible to minimise disruption to travellers, with passengers informed about developments by email or text message.
"Here we have a union leadership that is conducting a strike against our passengers, and that can't stand," Walther said.
A 2009 strike by cabin crew cost Lufthansa tens of millions of dollars.
In February this year, Frankfurt airport's apron control staff, who guide aircraft on the tarmac, walked out of their jobs over demands for higher pay.
Lufthansa has about 19,000 cabin crew and the UFO union represents about two-thirds of them.