There is a serious issue with terrorists claiming citizenship in a non-recognized state. The issue is that the Geneva Conventions states ("Conventions" is used in the singular throughout the document):
"Nationals of a State which is not bound by the Convention are not protected by it."
This means that members ("nationals") of ISIS, Al Qaeda, Hamas, etc (which have never ratified the Conventions and do not adhere to their rules), are NOT protected by the Geneva Conventions, which is (not "are") the principal document that defines war crime.
To be protected under the Conventions, one must wear an insignia and NOT carry concealed weapons.
These groups do not adhere to the basic minimum rules such as wearing of insignia and carrying weapons openly. Hence, those persons carrying weapons without insignia, or carrying weapons in concealment, may be shot after hearing by a "competent tribunal". In reality, any three officers can give them a five minute court martial and have them shot on the spot. This is a long military tradition.
Hence, it is not clear whether torture of such nationals is a violation of International Law. It is also not clear if such persons have a right to trial before being shot as "illegal combatants". It is not even clear whether such persons have any rights whatsoever.
The entire matter is very murky.