Russian forces are likely to have reached the centre of Bakhmut in their push to take over the city, according to UK intelligence.
Moscow’s troops have also seized the west bank of the river in the devastated area – endangering a key supply route to Ukraine.
Kyiv said Russia was concentrating all its efforts on capturing the eastern city, describing the situation as “difficult”, but said Ukrainian forces were holding out despite Russia’s numerical advantage during heavy fighting.
“Russia has made further gains and has now highly likely advanced into the town centre ... Ukraine’s key 0506 supply route to the west of the town is likely severely threatened,” the defence ministry tweeted.
Bakhmut, in the Russian-controlled Donetsk region, has been the focus of the bloodiest battles since Vladimir Putin’s invasion of Ukraine in February last year.
If confirmed, the MoD report suggests Ukrainian forces are under severe pressure, two days after president Volodymyr Zelensky said they would withdraw from the area if at risk of encirclement.
“Russian regular forces, likely including airborne troops, have probably reinforced the area, and Russia is again using artillery more effectively in the sector,” the MoD said.
Late last month the advance of Moscow’s troops had largely stalled.
Russia is now paying a high price for its advance, according to a top Ukrainian military official, eastern military command spokesperson Serhiy Cherevatyi.
The leader of Russia’s private Wagner militia, Yevgeny Prigozhin, said fighting was continuing in the west of the city.
“It must be said clearly that the enemy is not going anywhere,” he said on Telegram.
MoD officials said Wagner and Russian military commanders may have suspended their feud and improved cooperation. Signs of a revival of ammunition supply to the mercenary group have been reported.
Meanwhile, top officials in Washington said the Biden administration was investigating the leak of classified war documents about secret US and Nato plans to bolster the Ukrainian military ahead of an anticipated spring salvo against Russian forces.
The five-week-old documents reportedly reveal details of Ukraine’s requirements for a planned counteroffensive to reclaim territory taken by Russia, as well as how the US and Nato might face them.
According to military analysts who have seen the documents, certain parts appear to be modified and exaggerate the American tally of Ukrainians killed in the war while downplaying the estimates of Russian troops killed.
Seven civilians were reported on Thursday to have been killed by Ukrainian artillery strikes in Russian-controlled areas. And four civilians were killed in Donetsk when shells hit a car park, Russia’s Tass news agency said.
It came as French president Emmanuel Macron urged Chinese leader Xi Jinping to persuade Russia to negotiate an end to Europe’s most dangerous conflict since the Second World War.
Following talks in Beijing on Thursday with Mr Macron and European Union chief Ursula von der Leyen, Mr Xi said China was willing to “jointly appeal with France to the international community to remain rational and calm”.
