KYIV, Ukraine — A lightning Ukrainian offensive in the country’s northeast has reshaped what had become a grinding war of attrition. In a matter of days, Russian front lines have buckled, Moscow’s troops have fled and one village after another has come once more beneath Ukraine’s yellow and blue banner.

Ukrainian officials said on Saturday that their troops had taken the eastern city of Izium, a strategically important railway hub that Russian forces seized in the spring after a bloody, weekslong battle.