The source said the recently declassified U.S. intelligence report assessed that Russia began its full-scale invasion of Ukraine in February 2022 with 360,000 personnel.
Since then, the report found, 315,000 Russian troops, or about 87% of the total with which it started the war, have been killed or injured, the source said.
Those losses are the reason Russia has loosened recruitment standards for deployment in Ukraine, the source added.
"The scale of losses has forced Russia to take extraordinary measures to sustain its ability to fight. Russia declared a partial mobilization of 300,000 personnel in late 2022, and has relaxed standards to allow recruitment of convicts and older civilians," the assessment said, according to the source.
The Russian army began the war with 3,100 tanks, lost 2,200 of them and has had to "backfill" that force with T62 tanks produced in the 1970s, leaving it only 1,300 tanks on the battlefield, the source quoted the report as saying. Kyiv treats its losses as a state secret and officials say disclosing the figure could harm its war effort. A New York Times report in August cited U.S. officials as putting the Ukrainian death toll at close to 70,000.