KYIV, Ukraine—Ukrainian forces are seeking to roll back Russian gains as Moscow shifts its focus to controlling a swath of the country’s south and east.

Ukrainian forces said Sunday they drove Russian troops out of Trostyanets, in the northeast near the Russian border, potentially opening a road to the provincial capital of Sumy, which is encircled by the Russians.

Western officials see signs that Russia is consolidating its position to regain the offensive, and Ukrainian President Volodymyr Zelensky urged the West to provide his army with the heavier weapons it needs to fight the better-armed Russian military.

The retaking of Trostyanets comes after Moscow, having faced stiff resistance from the Ukrainians in its initial, multifront offensive, said Friday that it would refocus its campaign on the eastern Donbas region, where Russian forces hold a position of strength.

Russian forces have dug into defensive positions in the north and around the capital, Kyiv, which it has failed to seize. Russia’s firepower is currently concentrated on Mariupol, a strategically important city linking Russian-controlled parts of the Donbas with territory Moscow has captured in the south.

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.”

Ukrainian forces released this image on Sunday, saying it shows a Russian tank destroyed in Trostyanets.

Photo: General Staff of the Armed Forces of Ukraine/AFP/Getty Images

However, Western officials believe that Russia is now reinforcing in the Donbas region with fresh troops from the Wagner Group, a Russian mercenary organization, with the goal of encircling Ukrainian forces.

It isn’t clear how well trained these new Russian troops will be and whether they will have access to enough high-grade weaponry to make quick gains against battle-hardened Ukrainian troops there. The new battalion tactical groups come from Russia’s eastern military district, which experts say is the least battle trained and well equipped. However, refocusing the attack on a narrower front could solve some of the logistics problems that have dogged Russian forces and allow their dominant air power to assert itself.

Western officials estimate that as much as a fifth of the Russian force is no longer combat-effective and that morale is low. But they warn that the war is far from won for Ukraine.

“What we are not seeing is turning the tide, what we are seeing is some individual success,” one official said. The creation of new Russian battalion tactical groups indicates that Russian President Vladimir Putin is still going “all in,” the official said.

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

“This is an attempt to create North and South Korea in Ukraine,” Mr. Budanov said.

Cemetery workers on Sunday carried the coffin of a Ukrainian soldier who was killed in battle, during his funeral in Lviv.

Photo: ALKIS KONSTANTINIDIS/REUTERS

Firefighters worked Sunday at the site of a Russian missile attack on an oil depot in Lviv, a hub for people displaced by the war.

Photo: ALKIS KONSTANTINIDIS/REUTERS

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

Russian and Ukrainian negotiators will hold their next face-to-face meeting in Istanbul, Turkey said on Sunday. The office of Turkish President Recep Tayyip Erdogan said he urged Mr. Putin in a phone call Sunday to accept a cease-fire with Ukraine.

Western officials see little signs that Russia is willing to see a peaceful resolution to the conflict.

“No one thinks there is the chance of a diplomatic solution in the next few days or even few weeks,” said a senior European Union official. Mr. Putin is “going to keep on pushing and trying to overhaul” the Ukrainian government.

U.K. Prime Minister Boris Johnson said last week that even if a cease-fire is agreed to, the West must further arm Ukraine to strengthen “the quills of the Ukrainian porcupine as to make it in future indigestible to the Russian invaders.”

“This is just the beginning,” he said. “We must support a free and democratic Ukraine in the long term.”

During a visit to Warsaw on Saturday, Mr. Biden said the Russian leader’s invasion of Ukraine had ignited a “new battle for freedom” between democracies and autocracies. Mr. Biden also called Mr. Putin “a butcher” and appeared to call for his ouster. 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.

French President Emmanuel Macron said Sunday that he wouldn’t have used the same language Mr. Biden did because he is continuing to speak with Mr. Putin, including about a proposal to evacuate civilians from Mariupol.

Russian airstrikes left the town square of Bushiv severely damaged on Saturday.

Photo: Christopher Occhicone for The Wall Street Journal

With many stores closed, people came to sell basic wares on Saturday at a crossroads in Bilohorodka, on Kyiv’s outskirts.

Photo: Christopher Occhicone for The Wall Street Journal

“We want to stop the war Russia started in Ukraine without going to war,” Mr. Macron said in an interview on French television. “If we want to do that, we can’t escalate through words or actions.”

Asked about Mr. Biden’s remark on CNN’s “State of the Union,” Ukrainian ambassador to the U.S. Oksana Markarova said, “It is clear to us that Russia is a terrorist state, led by a war criminal.”

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.

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!”

NATO members have sent Kyiv large quantities of military, nonlethal and humanitarian assistance, but it 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 child waited on an evacuation train in Zaporizhzhia, southeastern Ukraine, on Saturday before heading west.

Photo: roman pilipey/Shutterstock

A couple kissed in a shelter on Saturday after an airstrike in Lviv.

Photo: Cheney Orr/Zuma Press

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.

Meanwhile, 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.

—Daniel Michaels, Warren P. Strobel and Laurence Norman contributed to this article.

Write to Isabel Coles at isabel.coles@wsj.com, Max Colchester at max.colchester@wsj.com and Yuliya Chernova at yuliya.chernova@wsj.com