Ukrainian forces said they have repelled Russian attacks in the east and recaptured some territory, even as Moscow moved to obstruct the flow of western weapons to Ukraine by bombarding railway stations and other supply-line targets across the country.

Heavy fighting also raged at the Azovstal steel mill in Mariupol that represented the last stronghold of Ukrainian resistance in the ruined southern port city, the Ukrainian military reported.

A Russian official earlier denied that troops were storming the plant, but the commander of the main Ukrainian unit inside said Russia’s soldiers had pushed into the mill’s territory.

“With the support of aircraft, the enemy resumed the offensive in order to take control of the plant,” the General Staff in Kyiv said, adding that the Russians were “trying to destroy Ukrainian units”.

To the west of Mariupol, Ukrainian forces made some gains on the border of the southern regions of Kherson and Mykolaiv, where Russian troops were reportedly trying to launch a counter-offensive, and repelled 11 Russian attacks in the Donetsk and Luhansk regions, the military said.

Five people were killed and at least 25 wounded in shelling of several eastern cities over the past 24 hours, Ukrainian officials said.

The Russian military said it used sea and air-launched missiles to destroy electric power facilities at five railway stations across Ukraine on Wednesday.

Artillery and aircraft also struck troop strongholds and fuel and ammunition depots.

Ukrainian foreign minister Dmytro Kuleba accused Moscow of “resorting to the missile terrorism tactics in order to spread fear across Ukraine”.

Responding to the strikes in his nightly video address, Ukrainian President Volodymyr Zelenskyy said: “All of these crimes will be answered, legally and quite practically – on the battlefield.”

The flurry of attacks comes as Russia prepares to celebrate Victory Day on Monday, marking the Soviet Union’s defeat of Nazi Germany. The world is watching for whether President Vladimir Putin will use the occasion to declare victory in Ukraine or expand what he calls the “special military operation”.

A declaration of all-out war would allow him to introduce martial law and mobilise reservists to make up for significant troop losses.

Belarus, which Russia used as a staging ground for its invasion, announced the start of military exercises on Wednesday. A senior Ukrainian official said Kyiv will be ready to act if Belarus joins the fighting.

The UK’s Ministry of Defence said the drills do not currently pose a threat to Ukraine, but Moscow is likely to use them “to fix Ukrainian forces in the north, preventing them from being committed to the battle for the Donbas”, the eastern industrial heartland that is Russia’s stated war objective.

The attacks on rail infrastructure were meant to disrupt the delivery of western weapons, Russian Defence Ministry spokesman Major General Igor Konashenkov said. Defence minister Sergei Shoigu complained that the West is “stuffing Ukraine with weapons”.

A senior US defence official said that while the Russians have tried to hit critical infrastructure around the western city of Lviv, specifically targeting railways, there has been “no appreciable impact” on Ukraine’s effort to resupply its forces. Lviv, close to the Polish border, has been a major gateway for Nato-supplied weapons.

Weaponry pouring into Ukraine helped its forces thwart Russia’s initial drive to seize Kyiv and seems certain to play a central role in the growing battle for the Donbas.

Ukraine has urged the West to ramp up the supply of weapons ahead of that potentially decisive clash.

Europe and the US have also sought to punish Moscow with sanctions. European Commission President Ursula von der Leyen called on the 27-nation bloc on Wednesday to ban Russian oil imports, a crucial source of revenue.

The proposal needs unanimous approval from EU countries and is likely to be debated fiercely. Hungary and Slovakia have already said they will not take part in any oil sanctions, and could be granted an exemption.

In Mariupol, mayor Vadym Boychenko said Russian forces were targeting the already shattered Azovstal plant with heavy artillery, tanks, aircraft, warships and “heavy bombs that pierce concrete three to five metres thick”.

“Our brave guys are defending this fortress, but it is very difficult,” he said.

Ukrainian fighters said on Tuesday that Russian forces had begun storming the plant, but the Kremlin denied it. “There is no assault,” Mr Peskov said.

Denys Prokopenko, commander of the Ukrainian Azov regiment which is defending the plant, said in a video that the incursions continued “and there are heavy, bloody battles”.

The United Nations announced that more than 300 civilians were evacuated on Wednesday from Mariupol and other nearby communities. The evacuees arrived in Zaporizhzhia, about 140 miles to the north west, where they were receiving humanitarian assistance.