After months of constant pressure and grinding, bloody advances, Russian forces are pressing up against multiple strongholds along more than 100 miles of the jagged front in the eastern Donetsk region of Ukraine. For Ukraine, losing any of those important defensive positions could significantly alter the contours of the fight for control of the region, long coveted by President Vladimir V. Putin of Russia.

Despite staggering casualties, Russian forces are mounting armored assaults and sending waves of infantry on foot, motorbikes and all-terrain vehicles to attack Ukrainian positions from Chasiv Yar in the north to the southern stronghold of Vuhledar, which is at risk of being encircled, according to Ukrainian soldiers and combat footage.

With attacks across fields strewn with their own dead, the Russians are racing to seize territory before the fall strips the foliage they use for cover and the rains turn fertile farmland into bog.

Even with both armies exhausted, the battles across the east remain as deadly as at any point in the war, according to Ukrainian soldiers and Western officials.

On each of two days last week, the Ukrainian military reported more than 200 clashes between the two sides — the highest such numbers in many months, according to DeepState, a group of analysts that maps the battlefield.