Russia’s army has captured most of Severodonetsk in eastern Ukraine, British intelligence suggests.
The key city in the Luhansk region, which had a prewar population of 106,000, has been the scene of street battles, which caused evacuation efforts to stall.
Defensive lines held by Ukrainian troops in the region for years have been breached by Russian forces as part of Moscow’s attempt to bring another city under its control.
Russia will probably turn its attention to gaining more territory in the neighbouring Donetsk region after they have fully secured Severodonetsk, Britain said.
“Russia has taken control of most of Severodonetsk,” the Ministry of Defence said on Twitter.
“The main road into the Severodonetsk pocket likely remains under Ukrainian control but Russia continues to make steady local gains, enabled by a heavy concentration of artillery.
"This has not been without cost, and Russian forces have sustained losses in the process.
“Crossing the Siverskyy Donets River, which is a natural barrier to its axes of advance, is vital for Russian forces as they secure Luhansk Oblast and prepare to switch focus to Donetsk Oblast.
“Potential crossing sites include between Severodonetsk and the neighbouring town of Lysychansk; and near recently captured Lyman. In both locations, the river line is probably still controlled by Ukrainian forces, who have destroyed existing bridges.”
Russia will probably require a “short tactical pause to re-set” before attempting to cross rivers to carry out attacks in the Donetsk Oblast, where Ukrainian soldiers are entrenched in defensive positions, the UK said.
While this tactic may seem necessary, any temporary pause in operations would mean the Russian army “risks losing some of the momentum they have built over the last week”, the ministry said.
On Wednesday, forces loyal to Kyiv appeared close to losing control of Severodonetsk.
The regional governor of Luhansk said Russian forces occupied about 70 per cent of the city.
"If in two or three days the Russians take control of Severodonetsk, they will install artillery and mortars there and will bombard more intensely Lysychansk," Sergiy Gaiday wrote on Telegram.
Mr Gaiday said Ukrainian forces were prepared to fight "until the end" in Severodonetsk, and that "street fighting continues".
Luhansk is one of two regions, with Donetsk, that make up Ukraine's industrial heartland, Donbas, which Russia has vowed to "liberate".
Earlier this week, Mr Gaiday suggested Ukrainian troops could make a tactical retreat from the city to reduce casualties and take up defensive positions on the outskirts to repel further Russian advances.
French cameraman Frederic Leclerc-Imhoff, 32, was shot dead on Monday while travelling in a lorry marked “humanitarian aid” near Severodonetsk. France has demanded an investigation into the incident.