Workers of Frankfurt airport, the third busiest airport in Europe, have been on strike since Thursday, causing hundreds of flights to be cancelled. And there may not be any end in sight either as the union representing the workers said they will uphold their strike for weeks, if necessary.

The GdF trade union, representing some 200 ground workers in Frankfurt airport, rejected a settlement to end the strike and renew their contracts. The workers, who guide planes in and out of their positions, demand higher pay, arguing their jobs are harder than ever before since the introduction of a fourth runway last October, according to Reuters. Specifically, the union is demanding 25 percent to 50 percent pay raises, depending on the worker's experience and position. They are also demanding bonuses and shorter work days.

We regret that we are forced to further measures, GdF union spokesman Matthias Maas said, according to AFP. Fraport AG's stubborn stance does not leave us other options.

Airport operator Fraport said that the increasing the the pay of the workers will only alienate others who earn less but do similar work. In order to compensate for the continuing strike, Fraport said that they will use non-union workers to operate about 70 percent of their scheduled flights.

We are ready for further talks with the union and are willing to compromise. We therefore urge the union to return to the negotiation table, Michael Peter Schweitzer said, a spokesman for Fraport.

About 1,300 flights fly out of Fraport airport. However, 172 were cancelled on Thrusday and 290 were cancelled on Friday. Most of the cancelled flights were operated by the German carrier Lufthansa, reporter Reuters. However, a Lufthansa spokesman said they will focus on getting their long-haul flights into the air on Monday.

 The workers had staged two shorter walkouts on Thursday and Friday. Airlines were able to operate about half of their flights, cancelling mostly their short to midrange flights, reported the Associated Press.