Ukraine's top commander Valery Zaluzhny was on Thursday removed from his post, in the biggest shake-up of Kyiv's military leadership since Russia's assault began almost two years ago.

The popular general had led the Ukrainian army since the start of the war, pushing back a vastly more powerful military force in the first months of the conflict.

But the failure of a much-vaunted counteroffensive last summer and public disagreement with Ukrainian leader Volodymyr Zelenskyy tarnished his reputation in the president's office.

"Today we had a frank discussion about what needs to change in the army. Urgent changes," Zelenskyy said in a statement on social media.

"I have offered General Zaluzhny to continue to be part of the team of the Ukrainian state. I would be grateful for his consent," Zelenskyy wrote.

Those comments came as Ukraine's defence minister – relatively new in his post – ended days of speculation over Zaluzhny's future by announcing on social media that he was being replaced.

Oleksandr Syrsky, who commanded Ukraine's lightning Autumn 2022 counter-offensive in the northeastern Kharkiv region, will take Zaluzhny's place, Zelenskyy said.

After over a year of deadlocked trench warfare, with Russia's army relentlessly pressing against outmanned Ukrainian soldiers across the sprawling front, Kyiv has sought urgent changes.

Zelenskyy called on his new military leadership to devise a strategy to beat back Russian forces.

"The year 2024 can be successful for Ukraine only if we make effective changes in the basis of our defence, which is the Armed Forces of Ukraine," Zelenskyy said. (AFP)