Russian and Ukrainian troops engaged in close-quarter combat in an eastern Ukraine city Sunday as Moscow’s soldiers, supported by intense shelling, attempted to gain strategic footholds in the region while facing fierce Ukrainian resistance.

Ukrainian regional officials reported that Russian forces were "storming" the eastern city of Severodonetsk, where the fighting has knocked out power and cellphone services and terrorized civilians who haven’t fled.

Severodonetsk, a manufacturing center, has emerged as an epicenter of Russia's quest to conquer Ukraine's industrial Donbass region. Russia also stepped up its efforts to take nearby Lysychansk, where Ukrainian officials reported constant shelling.

The two cities, with a combined prewar population of around 200,000, are the last major areas under Ukrainian control in Luhansk province, which makes up Donbass together with neighboring Donetsk. Russia is focused on capturing parts of both not already controlled by pro-Moscow separatists.

Russian forces made small advances in recent days as bombardments chewed away at Ukrainian positions and kept civilians trapped in basements or desperately trying to get out safely. Attacks to destroy military targets throughout the country also caused casualties in civilian areas

In his Saturday night video address, Ukrainian President Volodymyr Zelenskyy described the situation in the east as "very complicated’’ and "indescribably difficult." The "Russian army is trying to squeeze at least some result’’ by concentrating its attacks there, he said.

Severodonetsk Mayor Oleksandr Striuk said there was fighting at the city’s bus station on Saturday. A humanitarian center couldn’t operate due to the danger, Striuk said, and cellphone service and electricity were knocked out. Residents risked exposure to shelling to get water from a half-dozen wells, he said.

Some supply routes were functioning, and evacuations of the wounded were still possible, Striuk said. He estimated that 1,500 civilians in the city, which had a prewar population of around 100,000, have died from the fighting as well as from a lack of medicine and diseases that couldn’t be treated.

Haidai, the regional governor, claimed that the Russians had retreated "with losses" around the village of Bobrove, around 20 kilometers (12 miles) southeast of Sievierodonetsk, but were carrying out airstrikes in a nearby village on the strategic Siverskiy Donetsk river.

"The situation in Lysychansk is severe due to constant shelling by the occupiers, there are fatalities and wounded people," he wrote on Telegram, without elaborating.

On Saturday, he said, one civilian died and four were injured after a Russian shell hit a high-rise apartment building. A local cinema and 22 more residential buildings were also damaged, he said.

The Ukrainian military said Sunday morning that Russian forces were trying to strengthen their positions around Lyman, a small city that serves as a key rail hub in the Donetsk region.

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

Moscow claimed Saturday to have taken Lyman, but there was no acknowledgement of that from Kyiv authorities.

The Ukrainian army said that heavy fighting was ongoing around Donetsk, the provincial capital.

It also said that Russia launched an offensive Saturday night around the city of Bakhmut, in the neighboring Luhansk region, but had been pushed back.

In the same operational update, the military hinted at high levels of casualties sustained by Moscow, claiming that civilians were no longer admitted to hospitals in Russia-annexed Crimea as beds were needed by injured troops.

It was not immediately possible to verify the accuracy of these claims.

More widely, Russia launched renewed airstrikes overnight on Ukraine’s northern Kharkiv and Sumy regions, Ukrainian state agencies said.

The State Emergency Service of Ukraine emergency service said Sunday morning that Russian shelling caused fires around Kharkiv, the country's second-largest city. Russia has kept up a bombardment of Kharkiv, located in northeastern Ukraine after Ukrainian fighters pushed its forces back from positions near the city several weeks ago.

The Kharkiv regional prosecutor's office said a Russian shell broke through the room of a house and wounded a 50-year-old man and a 62-year-old woman early Sunday in the town of Zolochiv, around 40 kilometers (20 miles) northwest of Kharkiv.

The Ukrainian Border Guard Service said border areas in the Sumy region, east of Kharkiv, were hit with six unguided missiles. The agency did not mention reports of any casualties.