Londoner Aine Davis had been arrested in Istanbul in 2015 and is believed to be one of a four-man ISIS cell nicknamed "The Beatles", in reference to their British roots.
Jihadi John, killed in a US drone strike in Syria in 2015, was also a member of this cell.
According to the BBC, Davis told the judge that press stories claiming he was an accomplice of Jihadi John and behind the kidnapping and killing of hostages were untrue.
Davis is believed to have crossed the border from the terror network's stronghold of Raqqa last year and was detained by anti-terror police on November 12, 2015 - the day before coordinated ISIS terror strikes in Paris.
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Turkish officials have never released details of the charges against Davis and his alleged accomplices, and their trial took place largely in secret.
His wife, Amal El-Wahabi, was jailed in November 2014 for funding his terrorism following a trial at the Old Bailey court in London.