A Wizz Air Airbus 321 aircraft lands at the Chopin International Airport in Warsaw, Poland May 17, 2016. REUTERS/Kacper Pempel

BERLIN/FRANKFURT (Reuters) - The operator of Frankfurt airport, Fraport , is stepping up its drive to attract more low-cost carriers, with budget carrier Wizz Air set to start flights from Germany's largest airport, sources said.

Eastern European-focussed Wizz Air on Monday sent out an invite to a press conference at Frankfurt Airport, to be held on Tuesday morning, to discuss "expansion of its route network".

Two sources familiar with the matter said Wizz would start flying from Frankfurt this summer, with one person saying it would be a single route to the Bulgarian capital Sofia.

Low cost carriers have so far not taken major market share at Frankfurt, due to its high costs and long turnaround times. But with main customer Lufthansa not growing so quickly, Fraport is seeking new sources of passengers and in a surprise move last year signed up Ireland's Ryanair , which had previously ruled out flying from the German hub.

Wizz Air and Fraport declined to comment further.

(Reporting by Victoria Bryan and Peter Maushagen; Additional reporting by Marton Dunai; Editing by Harro ten Wolde)