Surely Ukraine is playing this very, very careful when it comes to taking to war to either Russia or Belarus. One stand out feature of this war has been the way Ukraine has done a pretty outstanding
PR job, the other is that much of that good
PR hinges on the evidence presented that it is wholly Russia that is the aggressor. They risk damaging that by overtly attacking Russian, or Belarusian, territory. In turn, that could result in a reduction of military aid to Ukraine, perhaps, as Ukraine might be viewed as a NATO proxy by some states.
There is some evidence that there are partisans within Russia, and, perhaps, Belarus, that are working on the side of Ukraine, either directly or indirectly because they have an interest in overthrowing Putin's regime. Until now there has been plausible deniability between the mysterious fires and explosions within Russia and the actions of Ukrainian forces. It has to be in Ukraine's best interests, in terms of maintaining wide international support, to stick to that tactic, rather than risk a long-range overt attack into either Russian or Belarusian territory.