The SACEUR says the masked men are playing for keeps. Those masked men are doing their own sort of massing at various decisive points, and the firing they've done so far has been mostly non-lethal. BTW, the "mass" principle of war is clear in US doctrine. Other military traditions have their own principles of warfare. Massing means concentrating forces in action, not sitting around, but the American media approaches military commentary without any nuance.
Partisan action against the Ukrainian government is focused on a few highly visible urban centers. This presents Ukraine with multiple opportunities to show media observers what massing really means if they ever get their act together. A sufficiently bloody recapture of a city hall or police station from the mysterious masked men of "Novorossiya" will give Russia the pretext it seeks to . . . mass its forces against Ukraine's forces.