The eastern Ukrainian city of Sievierodonetsk, which is crucial to Moscow's plans to conquer the industrial Donbas area, was overrun in part by Russian soldiers in a "frenzied thrust" on Tuesday, according to the city's mayor.

Oleksandr Striuk stated, "The city is being destroyed viciously block by block." He reported that severe street fighting continued and that artillery barrages threatened the lives of an estimated 13,000 civilians still taking refuge in the devastated city, which was once home to more than 100,000 people.

According to Serhiy Haidai, the governor of the Luhansk area, a Russian airstrike on Sievierodonetsk struck a tank of nitric acid at a chemical factory, causing a massive discharge of fumes. He tweeted an image of a large cloud hovering over the city and encouraged inhabitants to remain indoors and wear gas masks.

The majority of Sievierodonetsk city is under Russian hands, according to Haidai, who added that the city is not fully surrounded and that fierce battle is still ongoing.

Sievierodonetsk is crucial to Russian plans to seize the Donbas before more Western weapons come to bolster Ukraine's defense. Moscow-backed rebels had fought Ukrainian troops in the region for eight years and held territory even before the invasion.

Sievierodonetsk, located 145 kilometers (90 miles) south of the Russian border, is the last Ukrainian government-controlled pocket in the Luhansk region. The Donbas consists of the Luhansk and Donetsk regions.

Since Russia's invasion of Ukraine in late February, more than 1,500 individuals have died from various reasons, and evacuation efforts from Sievierodonetsk have been suspended due to bombardment.

"Civilians are dying from direct hits, fragmentation wounds, and under the rubble of demolished buildings," Striuk added, noting that the majority of residents are hiding in basements and shelters.

People are without electricity and require water, food, and medicine, according to the mayor, who added, "There are food supplies for several more days, but the question is how to distribute them."

In his nightly video message, Ukrainian President Volodymyr Zelenskyy described the situation in the Donbas as "very tough" because of Russia's deployment of "full fighting might" by its troops.

At least three people were killed and six were injured overnight in a Russian missile strike on the city of Sloviansk, west of Sievierodonetsk, according to a Facebook post by the governor of Donetsk, Pavlo Kyrylenko.

The road between two apartment complexes was blasted and badly pocked with shrapnel.