A nephew of the Turkish cleric Fethullah Gülen – the US-based exile whom the Turkish government accuses of masterminding a 2016 coup attempt – has been sentenced to seven-and-a-half years in prison on related terrorism charges, despite the fact the two have only met once.

Selman Gülen was found guilty in an Ankara court on Tuesday of being a member of his uncle’s Feto movement after prosecutors asked for a sentence of between seven-and-a-half to 15 years.

Selman Gülen denied the allegations, telling the court he had only met his uncle once in his life and that he believed he had been charged simply for being a relative of the cleric, who lives in self-imposed exile in Pennsylvania.

The younger Gülen was arrested in August 2016 at his home in Istanbul, where police said three mobile phones, hard drives and paper documents containing information related to Feto were seized.

Prosecutors also accused him of using encrypted Turkish messaging app ByLock to help plot the July 2016 military coup attempt that left 260 people dead.

Turkey maintains that ByLock was instrumental to the planning and execution of the failed coup. The state has used user profiles as evidence in thousands of terrorism and treason trials since, although human rights groups point out it was discontinued four months before the coup took place.

Almost 218,000 people have been held or tried for their alleged links to the attempt to oust Turkey’s ppresident, Recep Tayyip Erdoğan, the country’s interior ministry said last month. Around 150,000 state employees have been fired and independent media companies have been systematically shut down or bought up by government-friendly owners.

Ankara has repeatedly asked the US to extradite Fethullah Gülen for trial in Turkey – requests Washington has refused on the grounds of insufficient evidence. Gülen himself denies any involvement.

Turkish foreign minister Mevlut Çavuşoğlu said last week, however, that the US president, Donald Trump, had told Erdoğan on the sidelines of last month’s G20 summit in Buenos Aires that the US was “working on” extraditing Gülen and 80 of his supporters.

The White House later denied the claim.

Asked at a White House press briefing on Tuesday whether Trump had given any indication to Turkey that he was willing to extradite Gülen, spokeswoman Sarah Sanders said: “The only thing he said is that we would take a look at it. Nothing further at this point beyond that … Take a look at it but nothing committal at all in that process. Just that he would look into it.”

The cleric’s presence in the US is one of the major points of friction between the two Nato allies, but diplomatic efforts have been given a jump start following the death of Saudi dissident Jamal Khashoggi in the Saudi consulate in Istanbul in October.

The US has sought to insulate Riyadh from blame for the murder, while Erdoğan has maintained that the “highest levels” of the Saudi royal court are responsible for ordering Khashoggi’s murder.