The loss of younger officers was likely to exacerbate Russia's problems in modernising its military command and control, the ministry said.

"More immediately, battalion tactical groups which are being reconstituted in Ukraine from survivors of multiple units are likely to be less effective due to a lack of junior leaders," it said.

Russian forces intensified attacks on Monday to capture Sievierodonetsk, a key city in Ukraine's southeastern Donbas region which Moscow is targeting having failed to take the capital Kyiv early in the war.