German airline Lufthansa cancelled hundreds of flights today after cabin crew union UFO announced a walkout on flights from Frankfurt and Duesseldorf from 2pm  that could start the longest ever strike action at the carrier.

On the first day of a planned week of strike action over failed pension talks, flight attendants are being called to strike at Frankfurt, Lufthansa's biggest hub, and Duesseldorf until 10pm, affecting around 37,500 passengers.

The earlier Frankfurt-Malta-Frankfurt flight was not affected. 

Lufthansa said it had cancelled 290 flights planned including 15 long-haul departures, which are typically more profitable for carriers. Eight long-haul flights will still take off, it said.

More strike action can be expected tomorrow at Frankfurt, the union said, but Lufthansa's other main hub in Munich will not be subject to walkouts until after Sunday, given school holidays in that region.

Lufthansa Group airlines Swiss, Austrian Airlines, Germanwings, Eurowings, Brussels Airlines, Air Dolomiti and Cityline will not be affected by the strike. That means that overall, the airlines group can carry out 90 percent of the 3,000 flights it typically operates on a Friday.

The walkout comes after Lufthansa and the union failed to reach an agreement in a long-running row over early retirement benefits and pensions.

Lufthansa is trying to negotiate with various staff groups to bring down pension costs as part of a savings drive to allow it to better compete with low-cost rivals and leaner Gulf carriers.

Should Lufthansa management not make concessions, the union will carry out a threat to stage more walkouts until Nov. 13, it said.

"We regret this course of action, but we see no alternative," union head Nicoley Baublies said.

Lufthansa shares were up almost three percent on Friday, nearly recovering from Thursday, when they closed down 4 percent after the union confirmed it would strike.

Equinet analyst Jochen Rothenbacher said strike costs could total 20 million euros ($21 million) a day, depending on the number of cancellations, giving a total of around 140 million euros should the union strike on seven days, as threatened.

Strikes by pilots have already cost Lufthansa 130 million euros so far this year.