Russian forces bombarded the outskirts of Kyiv and a besieged city in northern Ukraine on Wednesday after promising to reduce attacks there in what the West dismissed as a ploy by Moscow to stem its heavy losses and regroup for other offensives.

Nearly five weeks into an invasion in which Russia has failed to capture any major city, the top UN human rights official said Moscow had bombed 50 hospitals as well as homes and schools across Ukraine in what may amount to war crimes.

Ukrainian officials called Russia out for...