The Russian supreme court has said that authorities are investigating at least 58 criminal cases against Russian nationals fighting alongside IS. The court's decision says that a Russian citizen is among the commanders of Jabhat al-Nusra. It is likely that the man is fighting with a faction named Seyfullakh Shishani's jamaat (fighting group), an Uzbek-led group that is part of Jabhat al-Nusra, although it maintains its own structure and consists mainly of Russian-speakers.
The Supreme Court material is only partially accurate and contains some claims that are, at best, rather odd.