The United States and Europe were planning new sanctions on Tuesday to punish Moscow over civilian killings in Ukraine, and President Volodymyr Zelenskiy warned more deaths were likely to be uncovered in areas seized from Russian invaders.

Russian forces withdrew from towns north of the capital Kyiv last week as it turns its assault to Ukraine's south and east. Ukrainian troops recaptured towns devastated by nearly six weeks of war, including Bucha, where dead civilians lined the streets.

Searing images of a mass grave in Bucha and...