The geneva conventions and other treaties that established what we call the international conventions on warfare were not written by he UN, amd the UN has no jurisdiction on them. The geneva convention was held in 1864, about 80 years before the UN was formed.

The ICRC is the jurisdiction in “charge” of defining warcrimes.

In any case, warcrimes are contextual. Bombing a power plant in one instance may be a legitimate target in war, while in another case, where Russias goal was to cause civilians to freeze and suffer, may very well be. However, I am not a lawyer of the international criminal courts.