You have two thing the Geneva Conventions and the International Criminal Court.
Syria is a member State party to the Rome Statute of the International Criminal Court from 29 November 2000 however they have not ratified the treaty along with 31 states. Interestingly the US, Israel and Sudan no longer intend to ratifty the treaty. The court only come into force if the country concern is unable to prosecute the individual concerned.
Syria is a signatory to the Geneva convention in 1953 and ratified Protocol 1 in 1983.
Quote:
Protocol I is a 1977 amendment protocol to the Geneva Conventions relating to the protection of victims of international conflicts, where "armed conflicts in which peoples are fighting against colonial domination, alien occupation or racist regimes" are to be considered international conflicts
They have not ratified Protocol II
Quote:
Protocol II is a 1977 amendment protocol to the Geneva Conventions relating to the protection of victims of non-international armed conflicts. It defines certain international laws that strive to provide better protection for victims of internal armed conflicts that take place within the borders of a single country.
These Syrian rebel are not party to the state, however crimes have been committed on both sides. It is unclear what category such combatants would charges under in which court.
I suspect you could only know the answer to that if it is pursued, and once you have stable enough state(s).