The Russian-battered eastern Ukrainian city of Sievierodonetsk appeared to be on the brink of becoming another Mariupol on Monday as the mayor told The Associated Press that Russian troops have entered, power and communications have been cut and “the city has been completely ruined.”

Moscow seeks to capture all of Ukraine’s industrial Donbas region, and Sievierodonetsk is key to that. Fierce street fighting is underway in the city as Ukrainian defenders are trying to push the Russians out, Mayor Oleksandr Striuk told the AP in a phone interview. Russian troops have advanced a few blocks toward the city center, he said.

“The number of victims is rising every hour, but we are unable to count the dead and the wounded amid the street fighting,” the mayor added. He said 12,000-13,000 civilians left in the city that once held more than 100,000 are sheltering in basements and bunkers to escape the Russian bombardment.

Russian forces stormed Sievierodonetsk after trying unsuccessfully to encircle it, and Ukrainian President Volodymyr Zelenskyy has described the situation as “indescribably difficult.” A Russian artillery barrage has destroyed critical infrastructure and damaged 90% of buildings. The mayor has estimated 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.

Sievierodonetsk, 89 miles south of the Russian border, has emerged in recent days as the epicenter of the Donbas fighting. Mariupol is the city on the Sea of Azov that spent nearly three months under Russian siege before the last Ukrainian fighters surrendered.

The Ukrainian military said Russian forces were reinforcing their positions on the northeastern and southeastern outskirts of Sievierodonetsk and bringing additional equipment and ammunition to press their offensive.

Luhansk regional Gov. Serhiy Haidai said the Russians also are pushing toward nearby Lysychansk. He said two civilians were killed and another five were wounded in the latest Russian shelling in the war.

Sievierodonetsk and Lysychansk span the strategically important Siverskiy Donetsk River. They are the last major areas under Ukrainian control in Luhansk, which makes up the Donbas together with the adjacent Donetsk region.

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.

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.

The Ukrainian army reported heavy fighting around Donetsk, the regional capital, as well as Lyman to the north, a small city that serves as a key rail hub in the region. “The enemy is reinforcing its units,” the Ukrainian armed forces’ General Staff said. “It is trying to gain a foothold in the area.”

Authorities in a Russia-backed separatist region said at least five civilians were killed in the latest Ukrainian shelling of Donetsk city including a 13-year-old boy.

Zelenskyy on Monday will address European Union leaders gathering in a new show of solidarity with Ukraine amid divisions over whether to target Russian oil in a new series of sanctions. He has repeatedly demanded that the EU target Russia’s lucrative energy sector and deprive Moscow of billions of dollars each day in supply payments.

Zelenskyy on Sunday visited soldiers in Kharkiv, Ukraine’s second-largest city, where Ukrainian fighters pushed Russian forces back from nearby positions several weeks ago. Russia has kept up bombardment of the northeastern city, and explosions could be heard shortly after Zelenskyy’s visit. Shelling and airstrikes have destroyed more than 2,000 apartment buildings since Russia invaded Ukraine on Feb. 24, according to the regional governor, Oleh Syniehubov.

In the wider Kharkiv region, Russian troops still held about one-third of the territory, Zelenskyy said.

Russian pressure also continued in the south on Monday. Russian Defense Ministry spokesman Maj. Gen. Igor Konashenkov said an artillery strike on the shipyard in the southern port of Mykolaiv destroyed Ukrainian armored vehicles parked on its territory.

In the Kherson region, the Russia-installed deputy head of the regional administration, Kirill Stremousov, told Russia’s Tass state news agency that grain from last year’s harvest is being delivered to Russian buyers, adding that “obviously there is a lot of grain here.” Ukraine has accused Russia of looting grain from territories its forces hold, and the U.S. has alleged Moscow is jeopardizing global food supplies by preventing Ukraine from exporting its harvest.

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. Petro Andryushchenko, posted a photo on the Telegram messaging app showing bodies stacked alongside closed supermarket counters. It wasn’t immediately possible to verify his claim.