Ukraine has sent reserve fighters from the Third Assault Brigade to Avdiivka as Russia tries to capture the town after months of heavy fighting.

The Brigade, which comprises assault infantry and is one of Ukraine's most prominent fighting groups, said the situation in Avdiivka was "hell" and "threatening and unstable," and that it had been sent in to reinforce Kyiv's troops there.

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 off Russian forces 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 in a statement on the Telegram messenger.

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

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

The Third Assault Brigade said it had conducted a raid against Russian forces in parts of Avdiivka and also inflicted heavy casualties. Reuters could not independently verify the statements.

The increasingly fraught Ukrainian position has fuelled speculation that Russia hopes to capture the town before a presidential election next month in which Vladimir Putin is set to be re-elected.

Military challenges

Nearly two years since Putin began the full-scale invasion that he calls a "special military operation," Ukraine's war effort is facing big challenges and uncertainty over the future of U.S. military assistance.

Senior military officials have said that tired troops, some of whom have been fighting for almost two years and are deployed along a sprawling 1,000-kilometer (620-mile) front, are facing shortages of artillery rounds.

President Volodymyr Zelenskyy replaced his popular army chief and other military leaders last week in the biggest military reshuffle of the war.

Ukraine has not said what its military strategy for the town is or whether it might eventually withdraw.

Avdiivka, where fewer than 1,000 residents are left of a pre-war population of 32,000, lies just to the 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 stopped functioning during the full-scale invasion.

Both sides regard Avdiivka as key to Russia's aim of securing full control of the two eastern "Donbas" provinces – Donetsk and Luhansk.

Avdiivka is seen as a gateway to Donetsk city, whose residential areas Russian officials say have been shelled by Ukrainian forces, sometimes from Avdiivka.