Nawaf Salam, the head of the International Court of Justice: “Israel must immediately hold its military offensive of any other action in the Rafah governorate which may inflict on the Palestinian group in Gaza, conditions of life that could bring about its physical destruction in whole or in part.”
The UN will want another International Tribunal for this but the mechanism to call one is performed via a security council vote. What they have to do first is make it so politically costly that the US won't veto it.
An International Tribunal called by the security council has binding powers above that of the Security Council itself. Effectively once called an international tribunal can ultimately bind the security council to act on its decisions.