More than 20,000 Russian troops have died and another 80,000 were wounded in five months of fighting in eastern Ukraine, particularly in Bakhmut, a White House official said Monday.

"We estimate that Russia has suffered more than 100,000 casualties, including over 20,000 killed in action," National Security Council spokesman John Kirby told reporters.

"Russia's attempt at an offensive in the Donbass, largely through Bakhmut, has failed... Russia has been unable to seize any really strategically significant territory," Kirby said.

Kirby, citing newly declassified US intelligence, said that about half of those killed were soldiers recruited by the private military company Wagner, which draws much of its ranks from prison populations in Russia.

The toll from fighting -- with the most intense battle being for Bakhmut, where Ukrainian troops have been driven from all but a sliver of the city -- accounts for losses since the start of December, according to the US figures.

"This attempted effort, particularly in Bakhmut, has come at a terribly, terribly high cost. Russia has exhausted its military stockpiles and its armed forces," Kirby said.

Kirby said he was not giving estimates of Ukrainian casualties because "they are the victims here. Russia is the aggressor."

The White House will not put "information in the public domain that makes it any harder" for the close Western ally, whose army is being armed and trained by a US-led coalition of countries, he said.