Russian forces are making progress in that region. Ukrainian military units have pulled out of Svitlodarsk, another city in Donbas. And earlier this week, Ukrainian President Volodymyr Zelensky said that up to 100 Ukrainian soldiers could be killed each day in the battle for Donbas.

But Russia is paying a steep price for the gains it has made. The Kremlin is sending units from southern Ukraine to fight in Donbas, according to Ukrainian officials, and losing so many men that continued Ukrainian resistance could eventually force it to shift strategies again.

Ukraine’s Defense Ministry said Wednesday morning that nine attacks had been repelled in the Donetsk and Luhansk regions, which together form Donbas that Russia is trying to take. Dozens of Russian vehicles were destroyed, the ministry said, including three tanks.

Mr. Haidai said that holding Severodonetsk would be crucial for Ukraine’s efforts to stop any further Russian advances, adding that the heavy casualties would eventually force Moscow to ease the assault.