Ukrainian President Volodymyr Zelenskyy has declared that the key eastern city of Lyman is "fully cleared" of Russian forces.

He made the declaration in a short video posted on social media on Sunday, a day after Russia's defense ministry had announced the withdrawal of troops from the city.

The defense ministry said on Saturday that its troops were withdrawn to more advantageous areas due to the threat of encirclement. It is unusual for Russia to reveal that its troops were withdrawn because of a "threat."

The announcement comes after Russian President Vladimir Putin unilaterally declared the annexation of four eastern and southern Ukrainian regions the day before.

The defensive move has provoked discontent even among Putin's staunchest supporters.

The head of the Chechen Republic in southern Russia, Ramzan Kadyrov, a close aide to Putin, strongly criticized the military on social media.

The founder of the Russian private military company Wagner Group, Yevgeny Prigozhin, echoed Kadyrov's criticism.

In its intelligence update on Sunday, the British defense ministry described Russia's withdrawal from Lyman as a "significant political setback."

Ukrainian troops are ramping up their counteroffensive in a bid to recapture territories occupied by Russia.

In a video released on Saturday, Zelenskyy said more Ukrainian flags will be flown in the following week in Donbas, which refers to the eastern regions of Luhansk and Donetsk.