The Ukrainian defence of the Donbas town of Bakhmut is "under increasingly severe pressure", the UK's Ministry of Defence (MOD) has said.
Officials in the besieged city say there is fighting on the streets, but Russia has not yet taken control.
British military intelligence reports "regular Russian Army and Wagner Group forces have made further advances into the northern suburbs of the city, which is now a Ukrainian-held salient, vulnerable to Russian attacks on three sides".
"Ukraine is reinforcing the area with elite units, and within the last 36 hours two key bridges in Bakhmut have been destroyed, including a vital bridge connecting the city to the last main supply route from Bakhmut to the city of Chasiv Yar.
"Ukrainian-held resupply routes out of the town are increasingly limited."
The head of Russia's Wagner Group private army claims they have almost completely surrounded Bakhmut, with just limited lines out.
Russia taking the city would represent a costly first victory for Moscow's winter offensive.
Bakhmut, which was once home to about 70,000 people, has been largely blasted to ruins following months of intense fighting.