Russian forces on Monday tried to press their offensive deeper into eastern Ukraine after taking full control of the eastern Luhansk region, the key stronghold, but Ukrainian President Volodymyr Zelenskyy vowed to regain the lost territory.

Ukraine on Sunday said the tactical withdrawal would save the lives of its soldiers, who would regroup to launch a counter-offensive with the help of long-range Western weapons.

But Moscow said the capture of Lysychansk less than a week after taking neighboring Severdonetsk meant it had "liberated" Luhansk. It said it would give the captured territory to the self-proclaimed Russian-backed Luhansk People's Republic, whose independence it recognized on the eve of the war.

The battlefield focus now shifts to the neighboring Donetsk region, where Kyiv still controls swathes of territory. Then Ukrainian General Staff said Russian forces were now focusing on pushing toward the line of Seversk, Fedorivka and Bakhmut in the Donetsk region. The Russian army has also intensified its shelling of the key Ukrainian strongholds of Sloviansk and Kramatorsk, deeper in Donetsk.

The Luhansk Governor Serhiy Gaidai, for his part, said that he expected the city of Sloviansk and the town of Bakhmut in particular to come under heavy attack as Russia tries to take full control of what is known as the Donbass in eastern Ukraine.

Russia says it has established full control over the Luhansk region following a withdrawal by Ukrainian forces from the bombed-out city of Lysychansk though Gaidai said fighting continued in two small villages.

"The loss of the Luhansk region is painful because it is the territory of Ukraine. For me personally, this is special. This is the homeland where I was born and I am also the head of the region," Gaidai said.

"In terms of the military, it is bad to leave positions, but there is nothing critical. We need to win the war, not the battle for Lysychansk," he said. "It hurts a lot, but it's not losing the war."

He said the withdrawal from Lysychansk had been "centralized" and orderly, and was necessary to save the lives of Ukrainian soldiers who were in danger of being surrounded.

"They (Russian forces) will not transfer 100% of their troops to some front because they need to hold the line. If they leave their positions, then ours can carry out some kind of counter-offensive," Gaidai said.

"If the commanders of our army withdraw people from certain points at the front, where the enemy has the greatest advantage in firepower, and this also applies to Lysychansk, it means only one thing," Zelenskyy said in his nightly video on Sunday.

"That we will return thanks to our tactics, thanks to the increase in the supply of modern weapons."

Zelenskyy said Russia was concentrating its firepower on the Donbass front, but Ukraine would hit back with long-range weapons such as the U.S.-supplied HIMARS rocket launchers.

"The fact that we protect the lives of our soldiers, our people, plays an equally important role. We will rebuild the walls, we will win back the land, and people must be protected above all else," Zelenskyy said.

Since abandoning an assault on the capital Kyiv, Russia has concentrated its military operation on the industrial Donbass heartland that comprises the Luhansk and Donetsk regions, where Moscow-backed separatist proxies have been fighting Ukraine since 2014.

Russian Defense Minister Sergei Shoigu informed President Vladimir Putin that Luhansk had been "liberated," the defense ministry said, after Russia earlier said its forces had captured villages around Lysychansk and encircled the city.

In Sloviansk, West of Lysychansk in the Donetsk region, Mayor Vadym Lyakh wrote on Facebook that fierce shelling had killed at least six people, including a 10-year-old girl, on Sunday.

Strikes on Kharkiv

Zelenskyy's office said Russian artillery strikes hit residential and farm buildings in the Kharkiv region.

Russia's defense ministry also said Sunday it had struck the military infrastructure of Kharkiv, Ukraine's second-largest city in the northeast, where a Reuters reporter said Ukrainian forces had been building fortifications after nightly shelling.

Outside a school in Kharkiv, some residents threw debris into a large crater created by an early morning rocket strike while others got help repairing damaged houses.

About 70 kilometers (44 miles) from Kharkiv on the Russian side of the border, Russia also reported explosions on Sunday in Belgorod, which it said killed at least three people and destroyed homes.

"The sound was so strong that I jumped up, I woke up, got very scared and started screaming," a Belgorod resident told Reuters, adding the blasts occurred around 3 a.m. (12 a.m. GMT).

Moscow has accused Kyiv of numerous attacks on Belgorod and other areas bordering Ukraine. Kyiv has never claimed responsibility for any of these incidents.

Meanwhile, Ukraine said its air force had flown some 15 sorties "in virtually all directions of hostilities," destroying equipment and two ammunition depots.

In the Russian-occupied southern Ukrainian city of Melitopol, Ukrainian forces hit a military logistics base with more than 30 strikes on Sunday, the city's exiled mayor Ivan Fedorov said. A Russian-installed official confirmed that strikes had hit the city.