The British defence ministry says the move shows Russian forces are under pressure and that the mercenaries are unlikely to make up for the loss of regular infantry units.
Russia has tasked mercenaries to hold sections of the frontline in Ukraine in a sign it is running short of combat infantry, Britain's Ministry of Defence has said, as Kiev steps up a counter-offensive in the south.
The British defence ministry said in an intelligence update on Friday that Russia is relying more on the forces from its private military company Wagner Group for frontline duties.
"This is a significant change from the previous employment of the group since 2015, when it typically undertook missions distinct from overt, large-scale regular Russian military activity," the ministry said on Twitter.
However, the ministry said the mercenaries are unlikely to make up for the loss of regular infantry units or alter the trajectory of Russia's military campaign.
Wagner and the Kremlin were not immediately available for comment outside regular business hours.
READ MORE: Deadly Russian strikes hit central Ukraine
Officials in Kiev said on Wednesday they had observed a "massive redeployment" of Russian forces to the south where British defence officials believe Russia's 49th Army, stationed on the west bank of the Dnipro River, is vulnerable.
The southern city of Kherson, key to Russia's overland supply lines from Russian-annexed Crimea, was now virtually cut off from the other territories occupied by Russia, British intelligence said on Thursday.
Kherson region fell to Russian forces soon after they began what Moscow calls "a special military operation" on February 24.
The Ukrainian military has used Western-supplied long-range missile systems to badly damage three bridges across the Dnipro in recent weeks, making it harder for Russia to supply its forces on the western bank.
Ukraine said its planes struck five Russian strongholds around Kherson and another nearby city on Thursday, the focus of its biggest counter-offensive of the conflict.
Also on Thursday, fifteen people were injured when Russian missiles hit military installations in Vyshhorod district on the edge of the Ukrainian capital, Kiev regional Governor Oleksiy Kuleba said.
Air raid sirens blared as Ukrainian President Volodymyr Zelenskyy addressed parliament alongside visiting Lithuanian President Gitanas Nauseda, as Ukraine marked its Day of Ukrainian Statehood with a public holiday for the first time on Thursday.
More than 10 Russian missiles also hit the Chernihiv region northeast of Kiev, regional Governor Vyacheslav Chausov told Ukrainian TV. Like Kiev, Chernihiv had not been targeted for weeks.
"This was Russia offering greetings on Ukraine’s Day of State Sovereignty," he said, adding there were concerns about a "second phase of ground operations by the enemy".
In addition, Donetsk regional governor Pavlo Kyrylenko said Russian forces shelled the town of Bakhmut. At least three people were killed and three were injured, he said.