According to Ukrinform, the UK Ministry of Defense said this in a new intelligence update published on Twitter.
The update states that since summer 2022, Russia has constructed some of the most extensive systems of military defensive works seen anywhere in the world for many decades. These defenses are not just near the current front lines but have also been dug deep inside areas Russia currently controls.
Imagery shows that Russia has made a particular effort to fortify the northern border of occupied Crimea, including with a multi-layered defensive zone near the village of Medvedivka.
Russia has also dug hundreds of kilometers of trenches well inside internationally recognized Russian territory including in the Belgorod and Kursk regions.
"The defenses highlight Russian leaders' deep concern that Ukraine could achieve a major breakthrough. However, some works have likely been ordered by local commanders and civil leaders in attempts to promote the official narrative that Russia is 'threatened' by Ukraine and NATO," the ministry said.