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.

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 maneuver 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.

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.

Reuters could not independently verify the statements.

The brigade took part in a counteroffensive in eastern Ukraine last summer and fought in the battle of Bakhmut, another town in eastern Ukraine that held out for many months before being captured last May.

“The enemy is continuing the active rotation of its troops (around Avdiivka) and is deploying new forces and equipment to the town,” the brigade said on the Telegram messenger.

“The situation at the moment the brigade was brought in was extremely critical.”

The unit’s deputy commander, Maksym Zhorin, said the fighting was much fiercer than the battle of Bakhmut and that Kyiv’s forces were outnumbered and outgunned in Avdiivka.

Russian forces have been trying to advance on the town since October and have surrounded it on three sides, leaving limited resupply routes for the Ukrainian troops dug in there.

Lykhoviy said Russia had concentrated around 50,000 troops on the Avdiivka front, and fighting in the city involved Russian tactical assault groups that were small but growing in size.

“Armored groups in the form of tanks and other armored vehicles are joining them,” he said.

Ukraine’s war effort is facing big challenges and uncertainty over the future of U.S. military assistance.

Tired troops, some of whom have been fighting for almost two years and are deployed along a sprawling 1,000-km (620-mile) front, are facing critical shortages of artillery rounds.

President Volodymyr Zelenskyy replaced his popular army chief last week and is set this week to visit France and Germany, where he is likely to seek support for urgent new military assistance.

Avdiivka, where fewer than 1,000 residents are left of a pre-war population of 32,000, lies just north of the Russian-held bastion of Donetsk which Ukraine lost control of in 2014 when Moscow’s proxies began an uprising.

The town has a vast coking plant that has stopped functioning during the war.

Both sides see Avdiivka as key to Russia gaining full control of Donetsk and Luhansk provinces and as a gateway to Donetsk city, residential areas of which have been shelled by Ukraine, sometimes from Avdiivka, Russian officials say.