Almost 300 flights were cancelled at Frankfurt, Europe's third largest airport, on Friday after a small group of ground crew resumed a strike over pay and warned the action could continue next week.

The GdF trade union called on 200 workers to strike after failing to reach a pay deal with airport operator Fraport, which says the workers' demands are too high.

A spokesman for the GdF union said while no strike action was planned for the weekend, they could resume action next week if Fraport did not meet their demands. They must give 24 hours' notice before any further strike action.

The union called the strike for 1400-2100 GMT (2 a.m.-9 p.m.) on Thursday and 0700-2100 GMT (7 a.m.-9 p.m.) on Friday. It wants higher pay because the workers' jobs have become more complex with the fourth runway that started operating in October.

Fraport has said the demands for increases in pay of up to 70 percent are irresponsible and called on the union to compromise.

It said it had signalled its willingness to return to the negotiating table but that it had not yet heard anything from the union regarding the possibility of more talks.

BEDS AND TEXTS

Frankfurt is Europe's third busiest airport behind London Heathrow and Paris Charles de Gaulle, serving over 56 million passengers in 2011 and employing over 70,000 people.

The strike means that around 290 out of the roughly 1,300 flights scheduled for Friday would not run, Fraport said, after 172 flights were cancelled on Thursday.

The vast majority of the flights cancelled were operated by German flagship airline Deutsche Lufthansa, which has sacrificed flights within Europe and Germany in order to keep long-haul traffic in the air.

Bankhaus Lampe analyst Sebastian Hein estimated the revenue lost by Lufthansa over Thursday and Friday at about 40 million euros ($52 million), while Metzler's Juergen Pieper said one day's strike could cost between 2-3 million in profits.

A strike by Lufthansa pilots in 2010 that was suspended after one day cost Lufthansa 48 million euros in lost revenues.

It's safe to assume that Lufthansa has purposefully taken the least profitable routes out, Hein said. The long-haul flights, which make the most money, have all been carried out.

Fraport said it was confident of being able to ensure at least 50 percent of flights on Friday. It is using operations management and non-union staff to fill the roles usually done by the striking workers, such as guiding planes to parking places.

Many passengers had already been rebooked onto new flights by airlines and tour operators before travelling to the airport, or had their tickets exchanged for Germany's high-speed trains.

Fraport said around 70 percent of flights operated on Thursday and no passenger had to spend the night in the airport's transit area. It and Lufthansa were ready to dust off the camp beds used by thousands of stranded passengers during the chaos caused by 2010's volcanic ash cloud.

Lufthansa also made available 600 hotel rooms in Frankfurt for Thursday night and sent almost 8,000 text messages to passengers.

So far, Fraport has not resorted to legal measures, such as temporary injunction, to avert the strike.

Court action was last year used to avert a planned strike by German air traffic controllers during the peak summer period and in 2010 pilots from Air Berlin and Lufthansa were forced by judges to call off strikes.

(Reporting by Victoria Bryan and Peter Maushagen; Editing by Jodie Ginsberg)