Yes it is. Inflicting causalities on civilians without any military justification or proportionality is a war crime. And against the Geneva Convention.
See the recent prosecution of Serbians by the World Court.
That's not inflicting casualties in and of itself. There have to be those additional factors, which aren't present here, since they are firing at the enemy forces, not at civilians for no reason. If a battle is happening in a populated area, some civilian casualties are unavoidable, and that is not a war crime.