More than 20,000 Russian combatants have died and another 80,000 were wounded in five months of fighting to capture the Ukrainian city of 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.

Kirby, saying he was citing newly declassified 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 in Bakhmut -- where Ukrainian troops are hanging onto only a small portion of the city -- accounts for losses since the start of December, according to the US figures.

"The bottom line is that Russia's attempted offensive has backfired after months of fighting and extraordinary losses," 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.