Russian troops are closing in on a key Ukrainian city in the east, after months of ferocious fighting. Putin's forces have spent over a year attempting to take the city of Pokrovsk, a key logistics hub for Ukraine's army on its eastern front.

It is seen by Russia as the "gateway to Donetsk" and its capture would severely impact Ukraine's ability to resupply its eastern army. The fall of Pokrovsk would also open the road to the other crucial fortress cities of Kramatorsk and Sloviansk. Kyiv's army has fought tooth and nail to prevent the Russians from seizing control of the city, but the situation is worsening for its heroic defenders.

Dr Marina Miron - an expert at the defence studies department at King's College London - said the Russians were cutting off access routes to the besieged city, using drones "to create a kill zone" and stop supplies from reaching Ukrainian troops.

"It will take time because what the Russians are trying to do essentially is to squeeze the Ukrainians out," Dr Miron told Sky News.

"They don't want to storm the city as it's too difficult and too manpower intensive - assuming a lot of losses."

Instead, they are trying to surround it completely, in what amounts to a change in tactics from previous city assaults.

In a further blow for Kyiv, Russian forces are reported to have advanced on Kupyansk in North-east Ukraine, according to the Institute for the Study of War. Kupyansk is also a key transport and logistics hub, where several major rail lines converge.

Earlier this week, Ukraine's military said Russia had moved personnel to the area via a pipeline.

However, officials insisted that the exit from the pipeline is under the control of Ukrainian defenders.

"A counter-sabotage operation is underway in the city, and search and strike operations are underway around the city," the Kyiv's General Staff said on Telegram on Saturday.

Kupyansk was recaptured by Ukrainian troops in their spectacular counteroffensive in the autumn of 2022.

The city, though, has been largely destroyed in the course of the war and continues to face attacks.