Just because a reference to the ICC is possible does not mean it would be effective. It would still require individuals to be captured, and enough evidence to be ascertained in order for a conviction.  Also, Russia has a track record of opposing the reference of the Syria case to the ICC, or any other involvement in the Syria issue. Negotiating with Russia could amount to a fruitless exercise  Also, such a reference could cause problems if a Western military intervention were to take place later, possibly exposing foreign peacekeepers to liability if any incident were to occur.