Ukraine Seeks to Exploit Shift in Russia’s Military Strategy

Ukraine said Sunday that its forces have retaken Trostyanets, about 14 miles from the Russian border, potentially opening a road to the provincial capital of Sumy, which has been encircled by the Russians since the early days of the invasion.

The successful counteroffensive shows how Ukrainian forces are seeking to take advantage of a shift in Russia’s military posture.

On Friday, after facing heavy losses and meeting stiff resistance from the Ukrainians, Moscow said it would refocus its offensive in Ukraine on the country’s east.

Russian forces have dug into defensive positions around towns and cities in the north and around the capital, Kyiv—which it has failed to seize—in order to concentrate on the Donbas region in the east.

Russia’s firepower is currently concentrated on Mariupol, a strategically important city linking Russian-controlled parts of eastern Ukraine with territory Moscow has captured in the south. Thousands have fled Mariupol, while those left behind endure constant Russian shelling and survive without adequate food, water or medical supplies.

Retaking Trostyanets “demonstrates that the Ukrainians are able to counterattack, which means Russia can’t assume that once they hold ground they have secured it,” said Jack Watling, an expert on land warfare at the Royal United Services Institute, a British think tank. “That limits the amount of resource they can apply to the place they are trying to take at any one time.”

Kyrylo Budanov, Ukraine’s head of military intelligence, said Russia sought to partition the country by merging territories under its control into a single statelet.

“After their failures near Kyiv and inability to overthrow Ukraine’s central government, Putin is already shifting his main operational directions—now it is the south and the east,” Mr. Budanov said. “In fact, this is an attempt to create North and South Korea in Ukraine.”

In territories under its control, Russia is seeking to establish parallel authorities and forcing people to reject Ukraine’s currency, the hryvnia, Mr. Budanov said.

The Russian pivot came a month into a war that has killed and wounded thousands of Ukrainians and uprooted more than 10 million from their homes.

During a visit to Warsaw on Saturday, Mr. Biden appeared to call for Russian President

Vladimir Putin’s

ouster, saying the Russian leader’s invasion of Ukraine had ignited a “new battle for freedom” between democracies and autocracies. A White House official later walked back Mr. Biden’s remark, which was dismissed by the Kremlin. A person familiar with the situation said that Mr. Biden’s comments weren’t part of his planned remarks.

While Mr. Biden was in Warsaw, Russian missiles struck a site roughly 210 miles away, near the western Ukrainian city of Lviv, which has become a hub both for people displaced by the fighting and for arms and other materiel the West is sending to support Ukrainian forces. Such support has helped Ukrainian troops regain some ground in areas where Russian troops had early success after the invasion began on Feb. 24.

The Russian strikes on Lviv damaged a plant used to repair and modernize Tor missile systems, radar systems and other equipment for the Ukrainian army, according to a Sunday briefing by Russian Defense Ministry spokesman Maj. Gen. Igor Konashenkov. 

The mayor of Lviv, Andriy Sadovyi, said the strikes had hit a fuel-storage facility and other infrastructure, and that military infrastructure had been removed from the city around the time the war began.

In a speech late Saturday, a visibly irritated Mr. Zelensky renewed his plea for tanks, planes and missile-defense systems. “This is what our partners have. This is what is covered with dust at their storage facilities,” he said.

“It cannot be acceptable for everyone on the continent if the Baltic states, Poland, Slovakia and the whole of Eastern Europe are at risk of a clash with the Russian invaders,” he said. “At risk only because they left only one percent of all NATO aircraft and one percent of all NATO tanks somewhere in their hangars. One percent! We did not ask for more. And we do not ask for more. And we have already been waiting for 31 days!”

Starting before the Russians invaded, members of the North Atlantic Treaty Organization have sent Kyiv large quantities of military, nonlethal and humanitarian assistance. The weaponry has recently increased in volume and variety, but still falls short of what Mr. Zelensky has publicly requested.

The U.S. and NATO allies have sent portable antitank and antiaircraft weapons, as well as lethal drones. Mr. Zelensky has requested fighter planes, tanks and antiaircraft systems capable of hitting Russian warplanes at high altitude, but said Ukraine hasn’t received what it needs.

A NATO spokeswoman Sunday cited comments by alliance Secretary-General

Jens Stoltenberg

on the same issue following a summit on Thursday. “We all listened very carefully to President Zelensky,” Mr. Stoltenberg said, declining to give details of systems being supplied. “But what I can say is that allies do what they can to support Ukraine with weapons so Ukraine can defend [itself].”

In a video, Taras Savchenko, the deputy head of the Sumy regional administration, showed the destroyed Russian tanks left behind in Trostyanets. On Sunday, the brigade involved in retaking the town said Russian forces had left behind weapons, equipment and ammunition that they would use to recapture other Ukrainian cities under Russian control.

Many facilities in the town, including a hospital, remain studded with mines, said Dmytro Zhyvytskyi, head of the Sumy regional administration.

Deliveries of medical supplies, food and other aid are being arranged, he said. Since communications in the town have been destroyed, rescue workers will drive around the town on Monday with loudspeakers to inform residents about the distribution of humanitarian aid.

Russia’s shifting war plan highlights how the Kremlin’s broader effort to take over most or all of Ukraine has stalled. When Moscow invaded Ukraine, it attacked along multiple fronts, aiming to take territory including Kyiv. In the opening days of fighting, Ukrainian troops were able to fend off an airborne assault to take the capital, halt tank columns heading to the city and forestall Russian advances in other areas.

—Yuliya Chernova and Daniel Michaels contributed to this article.

Write to Isabel Coles at isabel.coles@wsj.com