Right. But then we should be looking at international human law, and the principles of proportionality, distinction, and precaution, which are what - at least nominally - cover this. Which is more of an essay type question than it is a simple and very broad "what are they allowed to do," question.
As I'm not, hopefully understandably, going to write an essay on this myself, I'll link to one, from Médecins Sans Frontières:
https://guide-humanitarian-law.org/content/article/3/proportionality/
To the point, and to address part of your comment, I would agree with the statement there that trying to use "the overall strategic objective of its war to globally justify the alleged proportionality of massive civilian deaths and destruction" is wrong.