Vladimir Putin’s army has begun setting up “self-defence units” in Russian territory following a series of air strikes on its airfields, the British Ministry of Defence said on Wednesday.
A complex trench system is being dug by the Russian forces in Belgorod as the Kremlin promotes “defensive preparations within to burnish patriotic feeling”.
Russia is extending its defensive positions along its international border with Ukraine, and deep inside its Belgorod region, it is believed.
It comes after a third airfield was hit by a drone strike, a day after Ukraine demonstrated an apparent new ability to penetrate hundreds of miles into Russia with attacks on two air bases.
Ukraine has not commented on the strikes.
In its early morning briefing the MoD said: "On December 6, the governor of Belgorod announced he was establishing local ‘self-defence units’.
"Trench digging has been reported in Belgorod since at least April 2022, but the new constructions are probably more elaborate systems, designed to rebuff mechanised assault.
"There is a realistic possibility that the Russian authorities are promoting defensive preparations within internationally recognised Russian territory to burnish patriotic feeling.
"However, it probably illustrates some Russia decision-makers’ genuine (but false) belief that there is a credible threat of invasion by Ukrainian forces.”
It added that Russia's “critical weakness” was insufficient “strategic assessment in the central Russian government architecture”.
"Impartial official analysis is almost certainly frequently undermined by a tendency toward group-think and politically expedient conclusions," the MoD said.
US Secretary of State Antony Blinken has insisted that Washington "neither encouraged nor enabled" Ukraine to strike targets inside Russia.
He was speaking shortly after Moscow accused Kyiv of carrying out the drone attacks in its territory.
Putin has repeatedly warned the US, UK and other allies not to cross “red lines” by supplying long-range weapons to Ukraine.
The US-led Nato military alliance has ruled out providing such arms to Ukrainian forces, amid concerns that this could lead to a major escalation and nuclear war.
Mr Blinken said: “We have neither encouraged nor enabled the Ukrainians to strike inside of Russia, but the important thing is to understand what Ukrainians are living through every day with the ongoing Russian aggression against their country.”
Russian forces have been targeting Ukraine's energy and water supplies and several areas are without electricity.
The latest missile attack on Ukraine on Monday saw 70 rockets fired at targets across the country.
Four people were killed, officials said.
