It does sound borderline war crime, but then again the US buried thousands of Iraqi soldiers alive in the first gulf war, but our lawyers also did the mental gymnastics to justify it not being one beforehand so presumably they have some justification so its just barely legal. https://www.latimes.com/archives/la-xpm-1991-09-12-mn-2959-story.html

That being said, its not like Russia is being super careful about its rules of engagement.