The U.N. and a number of others have linked all sides in Syria's ongoing war to war crimes.
Rebel factions fighting against forces loyal to Syrian President Bashar al-Assad have been accused of taking hostages to extort money.
And ISIS has been accused of recruiting child soldiers, publicly executing civilians and using homemade chemical weapons.
One hundred and five countries voted to investigate possible war crimes, but 15 opposed the resolution.
That includes Syria. The country's ambassador to the U.N. called the investigation a "flagrant interference" in its internal affairs.