On a map, the gains hardly seem noticeable, measured in hundreds of yards, not hundreds of miles. But as President Trump presses Ukraine and Russia to make a deal to end their war, President Vladimir V. Putin of Russia is pushing to capture as much land as possible along a frontline that stretches about 750 miles, almost the distance from Chicago to New York.

As ever growing swarms of surveillance drones make any movement on the battlefield dangerous, the Russians are sending in small groups of soldiers on foot who are harder to detect. They effectively sneak past the Ukrainian troops, regroup and then attack, repeating this cycle as they inch forward.

These groups gained some territory earlier this month, especially in the crucial eastern Donetsk region near the embattled city of Pokrovsk. Larger formations of Russian troops also outflanked some Ukrainian defenses with drones and sheer numbers, raising fears that the frontline could start to crumble as the most concerted effort at peace talks in three years gets underway.

Ukrainian forces have largely pushed the Russians back from their recent gains and stabilized the front line after moving in reinforcements, according to interviews and battlefield maps. The fighting, however, remains intense. Gen. Oleksandr Syrskyi, Ukraine’s top military commander, said on Wednesday that Russian troops were stepping up their offensive actions in the north of Donetsk while continuing to push toward Pokrovsk.