Both Russia and Ukraine’s regular armies have seen so many soldiers killed and wounded that “the ability to generate and deploy reserve units to the front is likely becoming increasingly critical to the outcome of the war,” they added.
In its latest intelligence update, the Ministry of Defence in London said: “Heavy shelling continues as Russia pushes to envelop the Severodonetsk area via Izium in the north and Popasna in the south.
“Russia is highly likely preparing to attempt to deploy a large number of reserve units to the Donbas.”
It added: “The Russian authorities have not released the overall number of military casualties in Ukraine since 25 March.
“However, the self-declared Donetsk People’s Republic (DPR) publishes casualty figures for DPR forces. As of 16 June, the DPR acknowledged 2128 military personnel killed in action, and 8897 wounded, since the start of 2022.
“The DPR casualty rate is equivalent to around 55 per cent of its original force, which highlights the extraordinary attrition Russian and pro-Russian forces are suffering in the Donbas.
“It is highly likely that DPR forces are equipped with outdated weapons and equipment. On both sides, the ability to generate and deploy reserve units to the front is likely becoming increasingly critical to the outcome of the war.”
Britain, the US and other allies are fighting an information war against the Kremlin so their briefings need to be treated with caution.
However, Moscow’s claims are often even less believable, partly as the Kremlin does not admit to Mr Putin having launched a war in Ukraine on February 24.
The UK estimates that the invasion has led to the death of 15,000 to 20,000 Russian military personnel.
It has not given a figure for Ukrainian military losses.
However, Kyiv has admitted that during intense battles between 200 to 500 of its soldiers are being killed daily.
Thousands, if not tens of thousands, of civilians have also been killed, often in indiscriminate Russian shelling.
Russian forces have seized more villages near the industrial city of Severodonetsk in the eastern Luhansk province, according to reports on Wednesday.
But Ukrainian soldiers were still holding out in underground tunnels in the Azot chemical factory where 500 civilians, including dozens of children, are sheltering.
Fighting in the four month long war has favoured Russia in recent weeks because of its huge edge in artillery firepower, a fact Ukrainian President Volodymyr Zelensky acknowledged in a late Tuesday address.
“Thanks to tactical manoeuvres the Ukrainian army is strengthening its defences in the Luhansk region,” he said. “That is really the toughest spot. The occupiers are also pressing strongly in the direction of Donetsk.”
Luhansk and Donetsk provinces combined are known as the Donbas, where Russian-backed separatists have been fighting Ukrainian forces since 2014 following Russia’s annexation of Crimea from Ukraine.
“And just as actively as we are fighting for a positive decision by the European Union on Ukraine’s candidate status, we are also fighting every day for modern weaponry for our country. We don’t let up for a single day,” added Mr Zelensky, urging his country’s supporters to speed up arms deliveries.
The governor of Luhansk province, Serhiy Gaidai, said Russian troops were also advancing towards Lysychansk, attacking the buildings of police, state security and prosecutors, taking settlements and attacking the city with aircraft.
Some of the Russian offensive was reported to be from the south of the city, meaning its soldiers would not have to cross the Siverskyi Donets river.
Oleskiy Arestovych, an adviser to Mr Zelensky, said Russian forces could cut off Lysychansk, across the river Siverskyi Donets from Severodonetsk, from Ukrainian-held territory.
“The threat of a tactical Russian victory is there, but they haven’t done it yet,” he said in an online video.
Attacks have picked up in the Kharkiv region in the northeast, with at least 15 civilians killed by Russian shelling, civic chiefs said on Tuesday.
“Russian forces are now hitting the city of Kharkiv in the same way that they previously were hitting Mariupol - with the aim of terrorising the population,” Mr Arestovych said. “The idea is to create one big problem to distract us.”
The Ukrainian and Russian forces were entrenched in the eastern Ukrainian battlegrounds on Wednesday, a day of commemoration in both countries to mark the anniversary of Hitler's invasion of the Soviet Union in 1941.
June 22 is a significant date in Russia - the “Day of Remembrance and Sorrow” - marking when Hitler’s Nazi Germany forces invaded the Soviet Union in World War Two.
It is also commemorated in Ukraine and neighbouring Belarus, then part of the Soviet Union. The war there lasted 1,418 days from June 22, 1941, and historians estimate about 27 million Soviet soldiers and civilians were killed.
Meanwhile, in a symbolic decision, Ukraine is set to become an official candidate for European Union membership on Thursday, EU diplomats said.
Russia's failure to make a major breakthrough since invading Ukraine means time is on the side of Ukrainians, according to some military analysts.
"It's a heavyweight boxing match....there has not yet been a knockout blow. It will come, as RU forces become more depleted," retired US Lieutenant General Mark Hertling, a former commander of US ground forces in Europe, wrote on Twitter.