A German union called on its members Sunday to take part in a third warning strike this week at Frankfurt airport, Europe's third busiest, to ratchet up pressure for pay hikes.

The GdF union said in a statement that it was asking members to walk off the job for 24 hours from 0400 GMT Monday.

"We regret that we are forced to take further measures," it said.

"The rigid stance of Fraport AG leaves us no other choice than to continue our industrial action with all means at our disposal," it added, referring to the airport's operator.

Fraport said in its own statement that it was confident it could ensure that "up to 70 percent" of the around 1,250 daily flights at Frankfurt could be serviced despite the strike, including all intercontinental connections.

The wage dispute concerns some 200 so-called "apron control" staff who direct aircraft in and out of their parking positions, both in the control tower and on the tarmac.

The GdF union called for warning strikes after Fraport turned down mediation proposals in their wage dispute.

The strikes kicked off Thursday, grounding 172 flights, and intensified Friday with 280 flights cancelled, 250 of them from German flag-carrier Lufthansa, which uses Frankfurt as its primary hub.

The union halted industrial action over the weekend.

According to documents made available by Fraport, the union is demanding pay rises of 25-50 percent, depending on a worker's grade, as well as increased bonuses and reduced working hours.

Frankfurt airport is Europe's third-busiest after London-Heathrow and Paris Charles de Gaulle.

GdF repeatedly threatened strike action last year in a long-running wage dispute for regular air traffic controllers.

A strike was averted when the union and Germany's air safety authority DFS finally reached a deal in court in October.