According to a declassified US intelligence report, the war in Ukraine has cost Russia 315.000 soldiers killed or wounded, nearly 90 percent of the personnel it had at the beginning of the conflict, a source familiar with the intelligence said.
The report estimates that Russia's military modernization has been set back by 18 years due to Moscow's losses in manpower and armored vehicles.
The Russian Embassy in Washington and the Russian Defense Ministry did not respond to questions from Reuters reporters for comment on the report.
Russian officials say Western estimates of Russian casualties in the war are grossly exaggerated and almost always underestimate Ukrainian losses - which Russian officials say are enormous.
An intelligence community source says the report estimates that Russia began its invasion of Ukraine in February 2022 with 360.000 military personnel.
Since then, 315.000 Russian soldiers - or about 87 percent of the manpower since the start of the war - have been killed or wounded, the source said, adding that those losses are the reason Russia has been forced to relax recruitment standards and hire ex-prisoners or senior citizens to deployed them in Ukraine.
The Russian army was left without 1.300 armored vehicles on the battlefield and had to bolster its forces with T62 tanks produced in the 1970s, the source said.
Kiev treats the casualty data as a state secret, and officials say revealing the data would harm Ukraine's war effort.
A New York Times report from August cited estimates by US officials that the number of Ukrainian casualties was close to 70.000.
In the Ukrainian journal Tizhden, historian Yaroslav Tinchenko and volunteer Herman Shapovalenko reported last month that the "Book of Remembrance" project, using publicly available information, confirmed that 24.500 Ukrainian soldiers and civilians died in the conflict.
The real figure, they say, is much higher.
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