The ICC [1]. Like everything it's not perfect. But this might be in their jurisdiction. I don't know.
[1] https://www.voanews.com/a/icc-may-investigate-possible-war-c...
[1] https://www.voanews.com/a/icc-may-investigate-possible-war-c...
OTOH, ICC has no practical power to bring Russian personnel (other than those captured in combat by Ukraine) before it for trial, and the US “invade the Hague” law on the ICC might prove a model for Russia should the ICC gain custody of Russian personnel somehow.