The war for the Ukrainian city of Sievierodonetsk is horrific and will define the fate of the Donbas area, according to the country's president, as Russian troops raze the city in an offensive intended to seize control of eastern Ukraine.

In a Wednesday video statement, Ukrainian President Volodymyr Zelenskiy said, "This is a very brutal, extremely difficult combat, possibly one of the most severe of the entire war."

After failing to seize control of Kyiv, the Kremlin says it now seeks to "liberate" Ukraine's separatist-held Donbas region, where insurgents backed by Russia broke away from Ukrainian government rule in 2014. 

Before the February 24 incursion, rebels controlled around a third of the Donbas.

"Sievierodonetsk continues to be the epicenter of the conflict in Donbas... Currently, the majority of the fate of our Donbas is being decided there," he added.

Wednesday, Ukrainian forces in Sievierodonetsk retreated to the city's outskirts but pledged to continue fighting there for as long as possible.

The city in the Ukrainian province of Luhansk has been devastated by artillery bombardment. The regional governor of Luhansk, Serhiy Gaidai, stated that the city's core was being destroyed.

Ukrainian forces are holding out in the industrial zone of Sievierodonetsk. However, combat is occurring not only in the industrial zone, but also within the city of Sievierodonetsk itself, Gaidai stated on Ukrainian television on Wednesday night.

Gaidai said that Russian soldiers were destroying civilian structures in Lysychansk, the smaller twin city of Sievierodonetsk on the west bank of the Siverskyi Donets River. Ukrainian forces continue to hold the entire city.

The ambassador of Ukraine to the United States told CNN that Ukrainian forces were simply overwhelmed in Luhansk and Donetsk, which together form the predominantly Russian-speaking Donbas.

"But As we witnessed in the war for Kiev, it is possible to temporarily lose something. Obviously, we are attempting to limit this since we are aware of what might occur when Russians dominate territories, but we will regain it," Oksana Markarova said.

Gaidai stated that Russia now controls over 98% of Luhansk.

West of Sievierodonetsk in Sloviansk, one of the key Ukrainian-held Donbas cities, women with young children waited up Wednesday to receive help, while other locals carried buckets of water across the city.

Authorities estimate that about 24,000 civilians remain in the city, which is in the route of a planned attack by Russian forces assembling in the north.

The 85-year-old Albina Petrovna detailed the moment her home was attacked and her windows and balcony were destroyed.