Kyiv’s latest battlefield success has frustrated Vladimir Putin’s hopes of quickly seizing all of Ukraine’s Donetsk Region, a goal for the Russian president since he launched the full-scale invasion nearly five years ago.

The local counteroffensive, dubbed Vivaldi and first disclosed on Sept. 15, has already helped Ukraine regain 125 square kilometers (48 square miles) of territory, President Volodymyr Zelenskyy said during a visit to Ukrainian troops outside the town of Lyman on Wednesday. Those figures were confirmed by the Washington-based Institute for the Study of War.

Even if the advance, covering about twice the area of Manhattan, appears modest, it’s stymied Russian forces’ attempts to close in on the “fortress cities” of Slovyansk and Kramatorsk, the area’s last major settlements under Ukrainian control. Russian officials now privately accept that they won’t be able to capture the key cities until next year at the earliest, according to people close to the defense ministry in Moscow who spoke on condition of anonymity.

Operation Vivaldi allows Ukraine to demonstrate to its backers that it can still take initiative on the battlefield. That could give Kyiv additional cards in peace talks as US President Donald Trump pushes for a truce even though the Kremlin shows no sign of easing attacks. US envoys met separately with Ukrainian and Russian delegations in New York last month though there’s little sign of any progress in those talks.

Neither Ukraine nor Russia has achieved a sweeping battlefield breakthrough in recent years. Fighting has settled into slow-moving positional warfare as the proliferation of drones makes it increasingly difficult for either side to concentrate enough troops and equipment to conduct large-scale maneuvers.

To break through Russian defenses near Lyman, Ukrainian forces converted Soviet-era armored personnel carriers into robotic kamikaze vehicles and used them to destroy heavily fortified positions, Andriy Biletskyi, who commanded the assault, said. Newly-dug communication tunnels also let troops stay in contact even when Starlink satellite internet went offline, his unit said in a post on Telegram.

Intense Ukrainian drone strikes have also forced Russian troops to move fuel supplies and storage across the border into Russia’s Voronezh region after Kyiv’s forces severed a pipeline and targeted warehouses, he added.

Russia has demanded Ukraine surrender the entire Donetsk region as a prerequisite to any ceasefire, but Zelenskyy has ruled out ceding any of his country’s territory to Moscow. Putin has said the Russian army’s full seizure of the region is only a matter of time, though he’s repeatedly moved deadlines forward.

The Kremlin and Russian Defense Ministry publicly denied that Ukraine’s operation caused any setbacks, reluctant to contradict the Russian president’s claims of success on the battlefield.

If Russian troops can’t directly assault a city, they often attempt to surround it in a pincer movement like the claws of a crab, said Pavlo Narozhnyi, an army volunteer and military expert. “This operation severed one of those claws, causing the Russian tactic to fail. This success now secures the Sloviansk-Kramatorsk urban area,” Narozhnyi said.

Narozhnyi said the operation’s early secrecy was a reason for its success. But since it was revealed, Operation Vivaldi has been given higher-than-usual publicity compared to Ukraine’s other military operations. Biletskyi, commander of the Third Army Corps, regularly posts updates on its progress that are shared on a dedicated page on his unit’s website.

The optics of a battlefield success are also crucial on Ukraine’s domestic stage, giving Zelenskyy a rare positive message he can deliver to citizens who are increasingly anxious about the country’s preparedness for the coming winter.

Ukrainian forces, facing low stockpiles of air-defense missiles, must fend off more strikes by Russia’s new jet-powered drones on Kyiv and other cities. That’s raised public concern about the resilience of the country’s badly-damaged energy sector and other key infrastructure.

Biletskyi illustrated his Monday battlefield update with a video showing dozens of unarmed Russian soldiers lined up in a forest under Ukrainian guard. He said they were among more than 250 Russian troops captured in the operation, an unusually large number after years of grinding positional warfare.

The counteroffensive, named for the Italian Baroque composer best known for the Four Seasons, has now reached its third stage, Biletskyi said in a video statement on Monday. “Vivaldi’s music continues,” Biletskyi said.