There is some discussion from fringe Twitter users that Ukraine's artillery strike on a Russian forward command post that killed Russian senior leadership was a war crime, based on an entirely incorrect reading of international warfare laws concerning "treacherous killing", a situation which covers assassination through deception or ruse and doesn't apply here. After that was pointed out, the user in question pivoted to "The location of this forward command post was shared with Ukraine by the USA, which is an act of war or a war crime in certain circumstances", circumstances which were not expanded upon. However, I'm skeptical of this claim as well.
Is it an act of war or a war crime if, during a war between country A and country B, country C with friendly relations with country A but not officially involved in the war shares intelligence about country B with country A?