KYIV, Ukraine: Ukrainian President Volodymyr Zelensky on Tuesday said defending the Donbas was "vital" as the outcome of the battle for the eastern region would indicate the course of the war.

"Hanging in there in Donbas is crucial. Donbas is the key to deciding who will dominate in the coming weeks," Zelensky said on Telegram in his daily address to the Ukrainian people.

Russian forces have advanced in Donbas and control most of the Lugansk region, with fighting particularly intense in the city of Severodonetsk, the largest Lugansk city under Ukrainian control.

Capturing Severodonetsk and its twin city of Lysychansk would allow Russia to target Sloviansk further west in the Donetsk region.

Battles are also raging near Ukraine's second-largest city, Kharkiv, and Kherson in the south. Zelensky said Ukrainian forces were suffering "painful" losses in the northeastern Kharkiv region.

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"Fighting continues there and we need to continue fighting hard for full security in the Kharkiv region," he said.

"We continue pressuring the enemy in the south. Our key goal is to free Kherson."

The Donetsk and Lugansk regions, which make up the mainly Russian-speaking Donbas, were already partly controlled by pro-Moscow separatists since 2014.