"We've assessed, for the first time, that the Russians may be slightly below a 90% level of assessed available combat power," a senior defense official told reporters.
The drop in power is reflective of the 150,000 soldiers he amassed on Ukraine’s border in the lead-up to the invasion last month.
"It is not an assessment of all Russian military power," the senior defense official added.
Security officials continue to warn that Putin may be looking to bring supplemental forces into Ukraine to aid his war as Russian ground forces remain stalled across the country.
But there is scant evidence that any foreign fighters have headed to Ukraine despite reports that Putin has offered incentives to fighters from nations like Syria.
The senior defense official said there are indications of continued "discussions" by Moscow to aid its war "both in terms of resupply and also reinforcement," but there is no indication any such plan has been enacted.
Experts anticipate that if the Kremlin does move to send in reinforcements, it would pull support from troops stationed outside of Russia before pulling additional troops from inside its own borders.
"Of the battalion tactical groups that Mr. Putin has available to him across Russia, he's used about 75% in this particular operation," the senior defense official explained. "So I think our assumption would be he'd pull from outside the country first."
Ukrainian forces have continued to counter stiff Russian aggression. But despite weeks of shelling, Putin’s troops have not been able to make significant progress in advancing on the capital city of Kyiv.
"We're definitely seeing anecdotal evidence that the Ukrainians are not only defending well, where they choose to defend, but they are making efforts to take back territory," the senior defense official added.
Reports surfaced Tuesday showing that Ukrainian forces were able to retake a suburb outside of Kyiv, but the senior defense official said they were unable to verify the reporting.
Citing the Russian Ministry of Defense, a Russian tabloid claimed this week that 9,861 Russian troops had been killed in the fighting with another 16,000 injured – a stark difference from the 498 troops killed and 1,597 reported as injured by the ministry on March 2.
The report has since been taken down and the Pentagon has been unable to verify any causality estimates during the 27-day war.
But the senior defense official said it is not only the mounting number of deaths hindering Russian troops.
"They're having trouble. And we picked up indications that some troops have actually suffered and been taken out of the fight because of frostbite," the official said. "They are having continued logistics and sustainment issues."
The official said not only has Russia failed to properly plan for logistics and sustainment but also the "Ukrainians have done a good job frustrating their efforts."