More than 20,000 Russian soldiers have been killed in fighting in Ukraine since December, the US estimates.
A further 80,000 have been wounded, National Security Council spokesman John Kirby said, citing newly declassified intelligence.
Half of the dead are from the Wagner mercenary company, who have been attacking the eastern Bakhmut city.
Russia has been trying to take the small city since last year in a grinding war of attrition.
Moscow currently holds most of Bakhmut, but Ukrainian troops are still control a small portion of the city in the west. The fierce battle has taken on huge symbolic importance for both sides.
Ukrainian officials have also said they are using the battle to kill as many of Russia’s troops as possible and wear down its reserves.
