A war crime's a war crime. Doesn't matter if the Ukrainians did it, or the Russians, or Palestinians or Israelis. Or The UN or USA or NATO or Canadians. The only time a war crime's not a war crime is if it's a terrorist group doing it, because then it's terrorism instead of a war crime.
Terrorist is a fancy word for an enemy combatant that you don't like so much they don't get the POW status.
"Terrorism” and "war crimes” are overlapping categories, and being done by something independently defined as a “terrorist group” isn’t what defines something as “terrorism”, rather doing terrorism is what makes a group a terrorist group.