However, they believe that the Russian president is still reluctant to order a general mobilisation in Russia to provide more troops for his invasion which is believed to have already cost the lives of between 15,000 to 20,000 of his soldiers.
Three to four times as many Russian military personnel are estimated to have been wounded or incapacitated.
In its latest intelligence update, the Ministry of Defence in London said: “Over the coming weeks, Russia’s campaign will highly likely increasingly rely on echelons of reserve forces. These consist of several distinct components which Russia has almost certainly already started to field.
“Russia’s Combat Army Reserve is a recent innovation of part-time but volunteer reservists, which deploy as whole units typically ear-marked for rear area security tasks.
“The Human Mobilisation Resource is the sizable pool of all veterans who have served in the regular military in the last five years. Russian authorities are likely using volunteers from this category to fill out the third battalions within regular brigades.
“Despite a continued shortfall in the number of deployable reservists for Ukraine, the Russian leadership likely remains reluctant to order a general mobilisation.”
Mr Putin’s forces seized control in recent days of the industrial city of Severodonetsk in the eastern Luhansk province.
They are now trying to seize its twin city of Lysychansk, across the Siverskyi Donets river from Severodonetsk where Ukraine soldiers held out for weeks in a chemical plant where hundreds of civilians were also sheltering.
The MoD intelligence update added: “While Russia’s main operational focus remains the Severodonetsk-Lysychansk pocket, a week of consistently heavy shelling suggests Russia is now trying to regain momentum on the northern Izium axis.
“Ukrainian forces continue to hold the line in that sector, making good use of forested terrain to assist their defence.”
However, the TASS news agency quoted a separatist official on Sunday as saying that Moscow’s forces had entered Lysychansk from five directions and were isolating Ukrainian defenders.
The general staff of Ukraine’s armed forces said Russian forces were using artillery to try to cut off the city from the south but made no mention of separatists entering. Russian assault aircraft struck near the city, it added.
Serhiy Gaidai, the governor of the Luhansk province, said the city is suffering “catastrophic” damage from Russian shelling.
Severodonetsk fell to pro-Russian forces on Saturday.
Russian missiles struck a residential building in central Kyiv on Sunday, killing one and wounding six, officials said.
They were the first attacks on the capital in weeks and were condemned as “barbarism” by Joe Biden at a G7 summit in Germany where the US President, Boris Johnson and other world leaders were pushing for continued unity against Russia’s invasion.
Ukrainian presidential adviser Oleksiy Arestovych said missiles also struck the central city of Cherkasy on Sunday, hitting a strategic bridge linking the country’s west with eastern battlefields.
A missile strike in the Odesa region destroyed homes, causing a fire, and injuring six, including a child, said Serhiy Bratchuk, a spokesperson for the regional administration.