Ukrainian and Russian troops fought street by street for control of the industrial city of Sievierodonetsk on Monday in the pivotal battle of the Kremlin offensive in Ukraine’s eastern Donbas region.

Which side had the upper hand remained unclear. Russian forces had the numerical advantage and the situation was “difficult”, but Ukraine had “every chance” to fight back, its President Volodymyr Zelensky said after a regional official suggested Kyiv had lost ground.

Ukraine’s defence ministry said Russia was throwing troops and equipment into its drive to capture Sievierodonetsk, the largest remaining Ukrainian-held city in Luhansk province.

It has become the main target of the Russian offensive in the Donbas — made up of Luhansk and Donetsk provinces — as the Kremlin’s invasion grinds on in a war of attrition that has seen cities laid waste by artillery bombardments.

Ukrainian defenders had over the weekend pushed back the Russians as they seemed close to victory in Sievierodonetsk.

“Our defenders managed to undertake a counter-attack ... But now the situation has worsened a little for us again,” Luhansk governor Serhiy Gaidai told state television.

Sievierodonetsk mayor Oleksandr Stryuk said street fighting was raging and neither side was preparing to withdraw.

“The situation is difficult in the east,” Zelenskiy told a media briefing in the capital Kyiv. “We are in control of the situation, there are more (Russians), they are more powerful, but we have every chance to fight.

“If there is a (Russian) breakthrough in Donbas, it will be very difficult,” he added. Both sides say they have inflicted huge casualties on each other.

Russia says it is on a mission to “liberate” the Donbas - which has been partly held by separatist proxies of Moscow since 2014 — after Ukrainian forces pushed its troops back from Kyiv.