By Yuliia Dysa

President Volodymyr Zelenskiy said on Friday Russian forces have lost at least a brigade worth of troops trying to advance on Ukraine's eastern town of Avdiivka.

Russia renewed a push to encircle the embattled town in mid-October, trying to overwhelm Ukrainian positions with constant barrages of artillery and waves of troops and fighting vehicles, according to local and military authorities in Ukraine.

"The invaders made several attempts to surround Avdiivka, but each time our soldiers stopped them and threw them back, causing painful losses. In these cases, the enemy lost at least a brigade," Zelenskiy told British Prime Minister Rishi Sunak in a phone call, the president's office said in a statement.

Reuters could not independently verify the assertion and there was no immediate comment from Russia.

Brigades vary in size and can number between 1,500 and 8,000 troops. Battlefield losses are a state secret in Russia and Ukraine, though they are believed to run into the many tens of thousands on both sides since Russia launched a full-scale invasion of Ukraine in February 2022.

Russian military bloggers have reported territorial gains by Moscow's troops in the area, while Ukraine has described the situation as extremely difficult and said Moscow was throwing a huge number of forces into the assault.

Russia's offensive at Adviivka has become the battlefield focus nearly five months after Kyiv launched a counteroffensive in the occupied south and east. Kyiv has made slower progress than it wanted to due to vast Russian trenches and minefields, but it continues to report small gains in the strategic south.

The Institute for the Study of War, an American non-profit research group, said on Thursday that Russia had taken heavy losses in equipment near Avdiivka that would "likely undermine Russian offensive capabilities over the long term".

In a battlefield update on Friday, the Ukrainian General Staff said the military "steadfastly holds the defence and causes significant losses" to Russian troops, adding they were not giving up in their attempts to encircle the town.

Avdiivka, about 25 km (16 miles) from Russian-occupied Donetsk, is surrounded by Russian-held territory to the north, east and south, leaving only its west for Kyiv's troops to resupply and evacuate people.

The town, which is home to a vast coking plant, has been under attack since 2014 when conflict first broke out between Ukraine and Russian-backed forces. Vitaliy Barabash, head of Avdiivka's military administration, said the renewed attacks to capture the town were the largest since 2014.

(Reporting by Yuliia Dysa; Editing by Tom Balmforth and Jonathan Oatis)