By signaling its openness to a ceasefire, Ukraine has handed the Kremlin a difficult challenge at a time when the Russian military has the upper hand in the war: Should Moscow accept a truce and abandon hopes of making new gains,

Russia’s president, Vladimir V. Putin, dressed in fatigues, visited a command post near the front in Kursk late Wednesday to cheer on his military’s ejection of Ukrainian forces from much of the territory they had been occupying in the Russian border region.