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Ukrainian forces have retaken two villages near the bombarded city of Chernihiv which are located along main supply routes to Kyiv, British military intelligence said on Friday.
President Vladimir Putin “is now a man in a cage he built himself”, Defence Secretary Ben Wallace said.
Chernihiv and Kyiv, the capital, have been subjected to continued air and missile strikes despite Russian claims of reducing activity in these areas, the UK's Ministry of Defence (MoD) said.
“Ukraine has also continued to make successful but limited counter attacks to the east and north-east of Kyiv,” it said.
The retaking of Sloboda and Lukashivka villages, south of Chernihiv, could help reduce the attacks on Chernihiv.
A Russian official has said two Ukrainian military helicopters struck a fuel depot in the Russian city of Belgorod on Friday, the first accusation of a Ukrainian air strike on Russian soil.
Video images, so far unverified, of the purported attack posted online showed what looked like several missiles being fired from low altitude, followed by an explosion.
As the Russian war against Ukraine entered day 37, Mr Wallace said: “President Putin is not the force he used to be. He is now a man in a cage he built himself. His army is exhausted, he has suffered significant losses”.
“The reputation of this great army of Russia has been trashed. He has not only got to live with the consequences of what he is doing to Ukraine, but he has also got to live with the consequences of what he has done to his own army.
“We have seen it before. It always gets worse. It goes for more civilian attacks, more civilian areas.”
In an earlier intelligence update on Friday, the MoD tweeted that the Russian army was deploying up to 2,000 troops from Georgia into three tactical battle groups destined for Ukraine.
The UK Chief of the Defence Staff, Admiral Sir Tony Radakin, said Russia's bid to take all of Ukraine during its invasion looks to have “fallen apart” as Mr Putin's troops retreat.
He said Mr Putin was a “weaker and more diminished figure today” than he was before the invasion started on February 24.
He said the UK was “incredibly cautious” about believing Russian claims of ground troops withdrawing from Kyiv but said there did appear to be signs the Kremlin was preparing to focus its efforts on the east and south of Ukraine.