The UK says Moscow has had to deploy conscript troops to Ukraine despite Russian President Putin's assurances, even this week, that it wouldn't happen.
The UK briefing says: "As casulaties mount, President Putin will be forced to draw from across the Russian Armed Forces and other sources to replace his losses."
The UK also notes a "notable decrease" in Russian air activity over Ukraine in recent days, and says the Russian convoy north-west of Kyiv has made little progress and is "suffering continued losses at the hands of the Ukrainian Armed Forces".
About 5,000 Russian troops have been killed so far, reports had earlier estimated.