More than 20,000 Russian soldiers have died since December in its invasion of Ukraine, the US estimated on Monday.
The White House said it believes that Russia has suffered 100,000 casualties since December, some 20,000 of whom were thought to have died.
White House National Security Council spokesman John Kirby said the US estimate is based on newly declassified American intelligence.
In what has become a grinding war of attrition, the fiercest battles have been in the eastern Donetsk region, where Russia is struggling to encircle the city of Bakhmut because of Ukrainian resistance.
US officials previously said in November that Russia had suffered well over 100,000 killed or wounded in the first eight months of the war, but the new figures suggest that Russian losses have dramatically accelerated in recent months.
Mr Kirby said nearly half those killed since December are Wagner mercenary forces, many of them convicts who were released from prison to join Russia's fight.
He said the Wagner forces were “thrown into combat and without sufficient combat or combat training, combat leadership, or any sense of organisational command and control."
The White House has repeatedly sought to highlight the cost to Russia of Bakhmut, which it argues has limited strategic importance to the overall trajectory of the war.
Some analysts, however, believe that Russia wants to take Bakhmut because they think it could be helpful to efforts to advance on the larger cities of Kramatorsk and Sloviansk in the Donetsk region.
Mr Kirby said the Russian casualty count for "this little town of Bakhmut" was in line with some of the fiercest periods of fighting during World War II.
He added: “Russia's attempt at an offensive in the Donbas largely through Bakhmut has failed.
“Russia has been unable to seize any real strategic and significant territory."
Colonel General Oleksandr Syrskyi, the head of Ukrainian ground forces, said Russia continued to exert “maximum effort" to take Bakhmut but that it so far had failed.
“In some parts of the city, the enemy was counterattacked by our units and left some positions," he said.
White House officials declined to say how many Ukrainian troops have been killed or wounded in the fighting.
At a previous briefing in November that Ukrainian casualties - those injured or killed - were probably also about 100,000.
