POKROVSK, Ukraine — Russian and Ukrainian troops traded blows in fierce close-quarter combat Sunday in an eastern Ukrainian city as Moscow’s soldiers, supported by intense shelling, attempted to gain a strategic foothold to conquer the region. Ukraine’s leader made a rare front line visit to the city of Kharkiv as he sought to assess the strength of the country’s defenses.

Russian forces stormed Sievierodonetsk after trying unsuccessfully to encircle the strategic city, Ukrainian officials said, creating a situation that Ukrainian President Volodymyr Zelensky described as “indescribably difficult.” He said a relentless Russian artillery barrage had destroyed the town’s critical infrastructure and damaged 90 percent of its buildings.

“Capturing Sievierodonetsk is a principal task for the occupation force,” Zelensky said, adding that the Russians don’t care about casualties.

The mayor said the fighting had knocked out power and cellphone service and forced a humanitarian relief center to shut down because of the dangers.

The deteriorating conditions raised fears that Sieverodonetsk could become the next Mariupol, a city on the Sea of Azov that spent nearly three months under Russian siege before the last Ukrainian fighters surrendered.

Sievierodonetsk, located 89 miles south of the Russian border, has emerged in recent days as the epicenter of Moscow’s quest to capture all of Ukraine’s eastern industrial Donbas region. Russia also stepped up its efforts to capture the nearby city Lysychansk, where civilians rushed to escape persistent shelling.

The two eastern cities span the strategically important Siverskiy Donetsk River. They are the last major areas under Ukrainian control in Luhansk province, which makes up the Donbas together with the adjacent Donetsk region.

Zelensky, meanwhile, visited soldiers in Kharkiv, the country’s second-largest city, where Ukrainian fighters pushed Russian forces back from nearby positions several weeks ago.

“I feel boundless pride in our defenders. Every day, risking their lives, they fight for Ukraine’s freedom,” Zelensky wrote on the Telegram messaging app after the visit.

Russia has kept up its bombardment of the northeastern city from afar, and explosions could be heard shortly after Zelensky’s visit. Shelling and airstrikes have destroyed more than 2,000 apartment buildings in the city since Russia invaded Ukraine on Feb. 24, according to the regional governor, Oleh Syniehubov.

In a video address later Sunday, Zelensky praised Kharkiv regional officials but said he had fired the regional head of the country’s top security agency, the SBU, for his poor performance. In the wider Kharkiv region, Russian troops still held about one-third of the territory, Zelensky said.

After failing to seize Kyiv, the capital of Ukraine, Russia is focused on occupying parts of Donbas not already controlled by pro-Moscow separatists.

Russian Foreign Minister Sergey Lavrov told French TF1 television Sunday that Moscow’s “unconditional priority is the liberation of the Donetsk and Luhansk regions,” adding that Russia sees them as “independent states.”

He also suggested other regions of Ukraine should be able to establish close ties with Russia.

But in Luhansk, constant Russian shelling has created what provincial governor Serhiy Haidai called a “severe situation.”

“There are fatalities and wounded people,” he wrote on Telegram. On Saturday, he said, one civilian died and four were injured after a Russian shell hit a high-rise apartment building.

But some Luhansk supply and evacuation routes functioned Sunday, he said. He claimed the Russians had retreated “with losses” around a village near Sievierodonetsk but conducted airstrikes on another nearby river village.

Civilians who reached the eastern city of Pokrovsk, 80 miles south of Lysychansk, said they held out as long as they could before fleeing the Russian advance.

Yana Skakova choked back tears as she described leaving with her 18-month-old and 4-year-old sons while her husband stayed behind to take care of their house and animals. The family was among 18 people who lived in a basement for the past 2 1/2 months until police told them Friday it was time to evacuate.

“None of us wanted to leave our native city,” she said. “But for the sake of these small children, we decided to leave.”

Oksana, 74, who was too afraid to give her surname, was evacuated from Lysychansk by a team of foreign volunteers along with her 86-year-old husband.

“I’m going somewhere, not knowing where,” she wept. “Now I am a beggar without happiness. Now I have to ask for charity. It would be better to kill me.”

Sievierodonetsk Mayor Oleksandr Striuk said there was fighting at the city’s bus station on Saturday. Residents remaining in the city, which had a prewar population of around 100,000, risked exposure to shelling just to get water from a half-dozen wells, and there was no electricity or cellphone service. Striuk estimates that 1,500 civilians in the city have died since the war began, from Russian attacks as well as from a lack of medicines or treatment.

The Institute for the Study of War, a think tank based in Washington, questioned the Kremlin’s strategy of assembling a huge military effort to take Sieverodonetsk, saying it was proving costly for Russia and would bring few returns.

“When the battle of Sieverodonetsk ends, regardless of which side holds the city, the Russian offensive at the operational and strategic levels will likely have culminated, giving Ukraine the chance to restart its operational-level counteroffensives to push Russian forces back,” the institute said late Saturday.

In Mariupol on Sunday, an aide to its Ukrainian mayor alleged that after Russia’s forces gained complete control of the city, they piled the bodies of dead people inside a supermarket. The aide, Petro Andryushchenko, posted a photo on the Telegram messaging app of what he described as a “corpse dump” in the occupied city. It showed bodies stacked alongside closed supermarket counters.

“Here, the Russians bring the bodies of the dead, which were washed out of their graves during attempts to restore the water supply, and partially exhumed. They just dump them like garbage,” he wrote.

It was not immediately possible to verify his claim.

Regions across Ukraine were pummeled overnight by renewed Russian airstrikes. On the ground in the eastern Donetsk region, fighters battled back and forth for control of villages and cities.

The Ukrainian army reported heavy fighting around Donetsk, the provincial capital, as well as Lyman to the north, a small city that serves as a key rail hub in the Donetsk region. Moscow claimed Saturday to have taken Lyman, but Ukrainian authorities said their fighters remained engaged in combat in parts of the city.

“The enemy is reinforcing its units,” the Ukrainian armed forces’ General Staff said. “It is trying to gain a foothold in the area.”