Lots of propaganda by both sides.
Makes it hard to keep track of what is actually occurring.
Pro-Russian sources are squeeling right now and pointing fingers over the possible 600 dead Russkies right now mentioned in the OP, so it is not “propaganda.”
You have to work at it. We are fated to be “open source” intelligence analysts, sifting information sources, weighing one against another, seeking objective verifiable information (see Oryx, the photo-video database, for one). This gives you tools to judge plausibility of anything you hear.
This particular one I judge as “likely, but not certain”. The Ukrainians certainly are capable of reaching this target with effective munitions, and there is video from Russian sources of the shattered building. Whoever was in that place was extremely unlucky. The real questions are A. whether there were as many persons in it as claimed, or B. whether it was in fact some sort of accident. A magazine explosion in a barracks.
And, most necessary, aquiring a personal database of experience and deep reading of history and international affairs helps, to create a filter of perspective.