Moscow is redeploying forces to Ukraine from Russian-backed breakaway regions in Georgia, according to British defence officials.
The Ministry of Defence said that between 1,200 and 2,000 of these Russian troops are being organised into three battalion tactical groups.
“It is highly unlikely that Russia planned to generate reinforcement in this manner, and it is indicative of the unexpected losses it has sustained during the invasion.”
Earlier, Moscow said it would reduce its presence near the Ukrainian capital Kyiv, which it has failed to take after more than a month of fighting.
But Ukraine’s President Volodymyr Zelenskyy warned that Russia was instead moving towards a renewed push in the southeastern Donbas region.
He described the situation in the region – parts of which are controlled by Russian-backed separatists – as “extremely difficult”, adding that there are “battles ahead”.
Russian forces are also building up near the southern city of Mariupol, which has been targeted relentlessly over the past few weeks.
The city’s mayor said some 5,000 people have been killed there, with an estimated 170,000 people still trapped and struggling without food, water, or electricity.
Mr Zelenskyy said: “We know that (Russian troops) are moving away from the areas where we are beating them to focus on others that are very important – on those where it can be difficult for us.”
Russia has said it will concentrate on “liberating” Donbas, adding that the winding down of operations near Kyiv and the northern city of Chernihiv is to increase “mutual trust and create conditions for further negotiations”.
• US President Joe Biden announced the release of a record 180 million barrels of oil to cool crude prices that have increased due to the war
• Peace talks between Russia and Ukraine are due to resume today. There has been little progress so far
• The Russian defence ministry said it would open a humanitarian corridor from Mariupol to Zaporizhzhia today, according to Tass news agency
• Russia’s President Vladimir Putin ordered 134,500 new conscripts into the army, although the defence ministry says this has nothing to do with the war and is, instead, part of its spring draft
• Ukraine’s defence ministry said Russian forces killed 148 children during shelling and airstrikes since the invasion started in late February, although those numbers cannot be verified
Also late on Thursday, Ukraine’s state nuclear energy company Energoatom said Russian forces have left the Chernobyl plant.
The site of the world’s worst nuclear accident was taken by the Russians near the beginning of the war but, according to one employee at the site, the troops had been unaware of the danger still posed by radiation there.
Energoatom said the soldiers had received “significant doses of radiation” after digging trenches and other fortifications in the forest around the plant.
Signs of illness “showed up very quickly”, Energoatom said, adding that this had prompted the soldiers to “panic” and head for Ukraine’s border with Belarus.
Workers told Reuters news agency that a convoy had driven through the forest around the plant without radiation gear, which they described as “suicidal”.