FRANKFURT (Reuters) - German airline Lufthansa and airport operator Fraport are suing German trade union GdF over a series of strikes by airfield workers at Frankfurt airport in February, the companies said on Sunday.

The strikes by apron staff, who guide planes in and out of parking positions at the airport, resulted in the cancellation of around 1,800 flights, costing Lufthansa tens of millions of euros in lost revenue and leading Fraport to cut its guidance for passenger number growth.

The two companies said they had filed a complaint with a court in Frankfurt last week, but declined to give the amount of damages being demanded.

German magazine Spiegel, which said the German air traffic controllers' authority DFS and airlines Air Berlin and Condor had also filed complaints, said the total amount being demanded in compensation was 10 million euros ($12.3 million).

Condor, DFS and Air Berlin were not immediately available for comment.

The legal action comes after Lufthansa, Air Berlin and Ryanair failed last week in a bid to claim 3.3 million euros from the GdF, which mostly represents air traffic control staff, over threatened strikes by tower staff last summer.

($1 = 0.8132 euros)

(Reporting by Victoria Bryan; Editing by Susan Fenton)