KYIV, May 29 - 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 also made a rare frontline visit to Kharkiv, the country’s second-largest city, to assess the strength of the national defense, The Associated Press reported.

In the east, Russian forces stormed Sievierodonetsk after trying unsuccessfully to encircle the strategic city, Ukrainian officials said. President Volodymyr Zelensky described the situation there as “indescribably difficult,” with a relentless Russian artillery barrage destroying critical infrastructure and damaging 90% of the buildings.

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

The city’s 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 143 kilometers (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 of 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.

Zelenskyy, meanwhile, visited soldiers in Kharkiv, 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,” Zelenskyy 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.”

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 medicine or treatment. (ap/ez)