In its latest intelligence update, MoD analysts said Russia was using its superior numbers and firepower to “gradually seize territory” in and around the eastern Ukrainian city of Sievierodonetsk, where fighting has intesified in recent weeks.
But they warned its push to get more troops into Ukraine meant it was using combat reserves who would normally have been rested.
The update said: “Russia has likely started preparing to deploy the third battalion from some combat formations. Most brigades normally only commit a maximum of two of their three battalions to operations at any one time.
“The third battalions within brigades are often not fully staffed - Russia will likely have to rely on new recruits or mobolised reservists to deploy these units to Ukraine.”
They warned this would reduced the abilty of their armies to “regenerate combat power” after frontline service.
It comes as Ukrainian President Volodymyr Zelensky said no one knows how long the war will last.
Latest Defence Intelligence update on the situation in Ukraine - 12 June 2022
Find out more about the UK government's response: https://t.co/WiKk8bK2ge
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He praised his forces who he said were defying expectations by preventing Russian troops overrunning eastern Ukraine. In his nightly video address, Zelensky said he was proud of the Ukrainian defenders managing to hold back the Russian advance in the Donbas region, which borders Russia and where Moscow-backed separatists have controlled much of the territory for eight years.
He said: “Remember how in Russia, in the beginning of May, they hoped to seize all of the Donbas?”
“It’s already the 108th day of the war, already June. Donbas is holding on.”After failing to capture Ukraine’s capital Kyiv, Moscow focused on seizing the parts of the largely Russian-speaking Donbas still in Ukrainian hands as well as the country’s southern coast.
But instead of securing a swift, decisive takeover, Russian forces were drawn into a long, laborious battle, thanks in part to the Ukrainian military’s use of Western-supplied weapons.
Both Ukrainian and Russian authorities said Severodonetsk, an eastern city with a prewar population of 100,000, remained contested.
The city and neighboring Lysychansk are the last major areas of the Donbas’ Luhansk province not under the control of the pro-Russia rebels.