The UK Ministry of Defense said this in its latest intelligence update published on Twitter, Ukrinform reports.
According to the report, on December 5, 2022, multiple open sources reported explosions at Engels Airbase, in Russia's Saratov region, and at Dyagilyaevo airfield near Ryazan, south-east of Moscow. Two Tu-95 BEAR heavy bombers were reportedly damaged at Engels and three people were killed when a fuel tank exploded at Dyagilyaevo.
"The causes of the explosions have not been confirmed. However, if Russia assesses the incidents were deliberate attacks, it will probably consider them as some of the most strategically significant failures of force protection since its invasion of Ukraine," the ministry said.
According to the report, the sites are much deeper inside Russia than previous similar explosions: Engels is over 600km from Ukrainian-controlled territory.
Engels is the main operating base of Russia's Long Range Aviation (LRA) within western Russia and is home to more than 30 heavy bombers. These aircraft contribute to Russia's nuclear deterrent and have also frequently been used to launch conventional cruise missiles at Ukraine.
"The LRA is likely to respond by temporarily moving bombers to dispersal airfields. The Russian chain of command will probably seek to identify and impose severe sanctions on Russian officers deemed responsible for allowing the incident," British intelligence said.