Ukraine said on Thursday it was withdrawing troops from some parts of the eastern town of Avdiivka to better positions after months of heavy fighting, and battle-hardened reserve fighters from a crack brigade have joined the battle, Paralel.Az reports.

Russia is trying to encircle and capture Avdiivka nearly two years after its full-scale invasion of Ukraine and Kyiv's foothold in the town appears increasingly shaky, with its supply lines threatened.

Capturing Avdiivka is key to Russia's aim of securing full control of the two provinces that make up the industrial Donbas region, and could hand President Vladimir Putin a battlefield victory to hold up to voters as he seeks re-election next month.

"In Avdiivka a manoeuvre is underway in some places to withdraw our units to more advantageous positions, in some places to force (the Russians) out of positions," Ukrainian military spokesman Dmytro Lykhoviy said in televised comments.

"Therefore the key announcement with regards to all this is that supplies to Avdiivka and evacuations from there are difficult."

He said the military had activated a "reserve logistics artery" that had been prepared in advance.

"The situation on the front - Avdiivka, the east in general. We are doing everything possible to ensure that our soldiers have sufficient managerial and technological capabilities to preserve as many Ukrainian lives as possible," Ukrainian President Volodymyr Zelenskiy said in his nightly video address.

One of Ukraine's most prominent fighting units, the Third Assault Brigade, said it had been rushed to Avdiivka to reinforce Kyiv's troops there.

The brigade, which comprises assault infantry, said the situation in Avdiivka was "hell" and "threatening and unstable", but that it had conducted a raid against Russian forces in parts of the town and inflicted heavy casualties.