TURKEY has launched an espionage investigation after a discussion between top officials on potential military action in Syria was leaked on YouTube, heralding a possible government crackdown on its political opponents.

It is the most serious breach in weeks of highly sensitive leaks, which prime minister Tayyip Erdogan has blamed the Hizmet movement of US-based cleric Fethullah Gulen, a former ally, for running a "dirty campaign" of espionage to implicate him in corruption ahead of crucial municipal elections today.

Gulen denies involvement.