KYIV, Ukraine - 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 invasion 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 invading 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 Zelensky tarnished his reputation in the president's office.

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

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

Oleksandr Syrsky, who commanded Ukraine's lightning autumn 2022 counter-offensive in the northeastern Kharkiv region, will take Zaluzhny's place, Zelensky announced.

- 'Urgent' shakeup -

The announcement -- first made by the recently appointed defence minister -- ended days of speculation over Zaluzhny's future.

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.

New army chief Syrsky faces a myriad of challenges in his tray.

Large numbers of Russian forces are currently storming the frontline Ukrainian town of Avdiivka, while Moscow has intensified artillery strikes in the northeast Kharkiv region in a bid to force a Ukrainian withdrawal.

Announcing Zaluzhny's sacking, Zelensky 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," Zelensky said.

- The Iron General -

Dubbed the "Iron General" by Ukrainian media, Zaluzhny came to symbolise the country's resistance against Russia and enjoyed sky high approval ratings among the public.

He also garnered enormous respect among his troops, many of whom considered him a father figure.

While he had avoided the political spotlight, he is credited with spearheading some of Ukraine's most successful military campaigns, including the liberation of Kherson city in November 2022.

But his public comments -- to Western news outlets no less -- proved a source of constant consternation for Zelensky, already struggling to maintain unity over the issue of mobilisation.

In November 2023, Zaluzhny told The Economist that the conflict with Russia was at a "stalemate" and there would "most likely be no deep and beautiful breakthrough", an admission that Zelensky flatly denied.

Then, in an opinion piece for CNN exactly three months later, the 50-year-old said the army was bogged down by "regulatory framework" and called for urgent modernisation.

Zaluzhny said Ukraine would not be able to boost its army's manpower unless lawmakers took "unpopular" measures to mobilise more men.

But calls to mobilise half a million more people to swap out long-serving exhausted soldiers proved a highly divisive issue in a nation drained by fighting.