Tens of thousands of Russians are being sent to eastern Ukraine as part of an offensive planned after 15 February, according to the Ukrainian governor of Luhansk region.

"We are seeing more and more reserves being deployed in our direction," said Serhiy Haidai, who expected a three-pronged Russian advance.

Ukraine has warned repeatedly of an imminent offensive.

But there is widespread scepticism of significant Russian success.

The UK's defence intelligence briefing said Russia's aim was almost certainly to capture parts of Ukraine's eastern Donetsk region that were not already under occupation.

But the UK said "it remains unlikely that Russia can build up the forces needed to substantially affect the outcome of the war within the coming weeks".

Russian Defence Minister Sergei Shoigu said seven Ukrainians towns had been "liberated" in recent weeks and operations were progressing "with success" in two more, Bakhmut and Vuhledar.

For six months, Russian mercenaries -now joined by regular troops - have tried to capture Bakhmut, a small city with a pre-war population of 70,000.

Now a Ukrainian commander, Denis Yaroslavskiy, has said only 2,000 civilians remain and Russian forces have captured parts of the east and north of the city and continue to move forward.

The head of Russia's Wagner mercenary force said this week that Ukrainian forces were not in retreat anywhere in the city and fierce battles were taking place for every street, house and stairwell.

Ukrainian President Volodymyr Zelensky said the military was paying close attention to Russia's attempt to encircle Bakhmut. "We are countering them," he said on Monday night.

He announced that leaders with military experience were being appointed in border and front-line regions, who could "prove themselves" against current threats from Russia.

Ukrainian forces said on Tuesday they had shot down a Russian combat aircraft over Bakhmut, but there was no independent confirmation.

Russian forces have made little progress in Ukraine since their retreat from the major southern city of Kherson last November. Last month they captured the town of Soledar north of Bakhmut after an intense battle.

Almost a year into Russia's invasion, an estimated 300,000 Russian reserve troops have been recruited in recent months in an attempt to break through Ukraine's front lines in the east. Capturing Bakhmut could enable Russian forces to press on towards the bigger cities of Kramatorsk and Slovyansk.

Mr Haidai said the Russians' two-month training period was coming to an end and Moscow would need around 10 days to transfer them to the front for a new offensive. He suggested that in Luhansk region they would target three towns: Bilohorivka, Kreminna and Svatove.

President Zelensky has appealed to Western countries to hurry sending heavy weaponry to Ukraine to help Ukraine repel Russia's expected offensive. The US agreed last week to send long-range missiles that will enable Ukraine to double its attack range.

The Russian defence minister warned that Western heavy weapons were drawing Nato countries into the conflict and "could lead to an unpredictable level of its escalation".

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