German carrier Lufthansa (LHAG.DE) has already cancelled hundreds of flights a day because of the walkouts, which started last Thursday.

Just under 200 apron control workers, such as those who guide aircraft to parking places, are striking over a pay dispute with Fraport (FRAG.DE), which runs the airport.

The latest walkout started at 0400 GMT Monday and was due to continue until 0400 GMT Wednesday. It will now be extended to 2200 GMT Friday, the GdF union said on Tuesday.

Frankfurt, Europe's third-largest airport in terms of passenger numbers, usually has around 1,300 flights per day. Flights do not take off or land between 2200 GMT and 0400 GMT due to a night flight ban.

Fraport said more than 80 percent of flights took off on Monday. It estimated 197 flight cancellations so far out of a total 1,200 scheduled for Tuesday, meaning around 84 percent of services could still run.

Lufthansa, which accounts for more than half of flights at Frankfurt airport, said it expected to cancel only about 160 incoming and outgoing flights on German and European routes on Tuesday, after about 200 on Monday. The number of cancellations will drop further on Wednesday to around 100.

All intercontinental flights will take place this week, it said.

The first three days of the walkouts cost Fraport between 4.5-5 million euros in lost revenue, a spokesman said on Tuesday.

The airport operator has been running 70-80 percent of flights using staff from other areas of operations and has said it will not be forced to agree to what it what it said were "unreasonable" demands for pay increases of 50-70 percent.

Fraport has called on the GdF union to end the strikes and resume talks. The union has shown no sign of backing down, lengthening the strike periods each time and saying it could continue action for weeks.

The union on Tuesday accused Fraport of using unqualified staff and endangering the safety of the airport, which Fraport denied, saying safety was its highest priority.

The two sides have already resorted to mediation, with Fraport rejecting a proposal made by mediator Ole von Beust in early February saying it would go against the salary matrix within the company. The apron workers represent about 1 percent of its workforce.

"With the preparations taken, Fraport is able to hold out over long strike periods," the airport operator said on Tuesday.

(Reporting by Victoria Bryan; Editing by Kim Coghill, Dan Lalor and Jodie Ginsberg)

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