Kyiv says its forces are still advancing, after claiming to have seized more than 440 square miles of Russian territory in the surprise assault launched two weeks ago that has shaken up the dynamics of the war and posed a new challenge to the Kremlin.

It appears to have taken Russia completely off guard after months focused on a push in Ukraine’s east and the rest of the 600-mile frontline.

Russia’s defense ministry said its forces seized another settlement in the embattled Donetsk region…