Fethullah Gulen
Islamic preacher Fethullah Gulen is pictured at his residence in Saylorsburg, PennsylvaniaReuters

Turkey's prime minister Recep Tayyip Erdogan has said that Ankara is to begin the legal process for the extradition of influential cleric Fethullah Gulen, who is in self-imposed exile in Pennsylvania.

Erdogan accused the 70-year-old former ally - who commands a global empire of media outlets, private schools and charities - of trying to topple his government through a graft probe and to have built a parallel state in Turkey, infiltrating police and the judiciary.

In an interview with the Charlie Rose show to be aired on Bloomberg TV, Erdogan said that "it's sad for us to see such a group [the Gulen movement] can exist in the US" and that Turkey has "expectations from our model partner [the US]".

Tensions between the reclusive Muslim cleric and Erdogan, who were previously allies, have boiled over in recent months, and ultimately exploded after the Turkish government announced plans to outlaw private schools, including those run by the Gulen movement - also known as Hizmet (Service).

The movement, which proposes a moderate and modern interpretation of Islam, has extensive influence in the Turkish police and judiciary. Gulen members are believed to have infiltrated the secret services, law enforcement offices and the AK party itself.

Erdogan blamed Gulen for a wave of high-profile arrests that shook his moderate-Islamist AKP government.

Forty-seven businessmen and politicians, including the sons of three Cabinet ministers, have been put in custody on corruption and bribery charges.