Casualty estimates for militaries on both sides are highly speculative, U.S. officials have said. They often give ranges rather than specific numbers, though just last week, the C.I.A. director estimated that 60,000 Russian soldiers had been killed or injured. And some estimates have gone as high as 80,000 casualties.

If the Biden administration’s current estimate is accurate, it represents a staggering toll. Estimates of the number of Russian forces in Ukraine ranged as high as 150,000 in the spring, meaning that roughly half could be out of action.

Pentagon officials have said that losing just 10 percent of a military force, including both those killed and injured, renders a single unit unable to carry out combat-related tasks. Such losses also affect the morale and cohesion of a military unit.