Yes. It is a human shield tactic, which is prohibited by the international law of war. There is no way to discern the presence or absence of civilians when the engagement is by artillery. And the Russian forces are blameless if they fire on the Ukrainian forces, regardless of civilians nearby (since the Ukrainians should have kept their forces segregated from civilians).

If the Ukrainian forces need a place to pause and sleep, they can pitch tents or build expedient shelters outside the city.