Russia will need to conduct a contested river crossing or advance on its stalled flanks after all the main bridges over the Siverskyy Donets river, linking Sievierodonetsk and Ukrainian-held territory, have likely been destroyed, Britain's Defence Ministry said on Thursday.

Ukraine has probably managed to withdraw a large proportion of its combat troops, who were originally holding the town, the ministry said.

"Russia's combat force in the Donbas is highly likely operating in increasingly ad hoc and severely undermanned groupings," it said in an intelligence update posted on Twitter.

After being pushed back from the outskirts of Kyiv in May, Russia's troops have focused their efforts on capturing the entirety of the Donbas, an industrial region where fighting has focused in recent weeks on Sievierodonetsk, now largely in ruins.

(Reporting by Akanksha Khushi in Bengaluru Editing by Mark Potter)