The Russian military has expanded its control of territory in eastern Ukraine in an offensive that could cut supply lines and encircle frontline Ukrainian forces, British and Ukrainian military chiefs said.

The UK's Ministry of Defence said that Ukrainian forces had withdrawn from some areas near the city of Lysychansk, the latest major battlefield in Russian President Vladimir Putin's war against Ukraine, to avoid the possibility of being encircled as Russians sent in reinforcements and concentrated their firepower in the area.

Ukraine's General Staff said Russian forces took control of the villages of Loskutivka and Rai-Oleksandrivka, and were trying to capture Syrotyne outside Sievierodonetsk.

For weeks, Russian forces have pummelled Sievierodonetsk, the administrative centre of the Luhansk region, with artillery and air raids, and fought the Ukrainian army house-to-house.

Ukrainian forces remain holed up at the Azot chemical plant on the city's edge, where about 500 civilians were also sheltering.

The Russians are also pressing their offensive on Lysychansk, which is located on a steep river bank facing Sievierodonetsk.

The MoD noted in its intelligence assessment on Thursday that Russian forces have probably advanced more than three miles towards the southern approaches of Lysychansk since Sunday.

"Some Ukrainian units have withdrawn, probably to avoid being encircled," the statement said. "Russia's improved performance in this sector is likely a result of recent unit reinforcement and heavy concentration of fire."

Ukraine's military said the Russians were also moving to overtake the hills overlooking a highway linking Lysychansk with Bakhmut, to the south west, in an attempt to cut the supply lines of Ukrainian forces.

Following a botched attempt to capture the Ukrainian capital in the early stage of the invasion on February 24, Russian forces have shifted focus to Ukraine's eastern industrial heartland of Donbas, where Ukrainian forces have fought Moscow-backed separatists since 2014.

The Russian military currently controls about 95% of the Luhansk region and about half of the neighbouring Donetsk region of Donbas.