Russian forces on Thursday pounded an eastern Ukrainian city and the two sides waged pitched street battles that Ukrainian President Volodymyr Zelensky said could determine the fate of the critical Donbas region.

In the 3-month-old war, Russia set its sights on the industrial Donbas region of coal mines and factories, where Moscow-backed separatists have been fighting Ukrainian troops for years and where they already held swaths of territory before the invasion.

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But, the Russian advance has not been as quick as expected, and the plodding battle for control of Severodonetsk has devolved into street-to-street fighting that has been relatively rare in the conflict.

Speaking to The Associated Press, Luhansk governor Serhiy Haidai said, “Fierce battles continue in the city itself, street battles are taking place with varied success in city blocks.

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Sievierodonetsk, which became the administrative capital of the Luhansk region after the original one was taken by separatists in 2014, is the last pocket of the region that Russia has not yet claimed control.

Zelensky called the painstaking fight for Severodonetsk the “epicentre” of the battle for the larger Donbas, which comprises Luhansk and Donetsk.

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“In many ways, it is there that the fate of our Donbas is being decided,” Zelensky said on Wednesday in his nightly video address, which was recorded in the street outside his office in Kyiv.

Analysts have suggested that Russia’s slow advance in the Donbas could eventually open up the possibility of a negotiated settlement to the war.

In other developments, Haidai said Russian forces are also targeting Lysychansk, the city that neighbours Severodonetsk, with “day and night shelling,” as well as trying to storm a key road leading from Lysychansk to the southwest.

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On Thursday, Russia claimed that it struck a training facility west of the capital, far from the front lines. Russia’s Defence Ministry said it used air-launched missiles against a Ukrainian military base in the Zhytomyr region where it alleged mercenaries were being trained.

There was no immediate response from Ukrainian authorities to the Russian claims. Moscow has repeatedly accused Ukraine of using mercenaries in the fighting.