Besiktas are looking to strengthen their side in the January transfer window.
The Black Eagles are performing well both domestically and in Europe this season and want to add depth to their squad.
26-year-old Demirok plays at centre-back for his club and has made 10 appearances this season.
Serdar Aziz has been linked with all three of the big Istanbul clubs over previous years and has made nine appearances in defence for Bursaspor this season.
Bilic’s side are also in the market for a new right back and have made an approach for both Dinamo Zagreb’s Ivo Pinto and Molde’s young right back Marin Linnes.
Besiktas are currently third in the Turkish Super League with 17 points from nine games and are on the verge of qualifying to the next round in the Europa League.