WASHINGTON — The White House said Monday it now estimates that just since December Russia has suffered 100,000 casualties, including 20,000 killed, as Ukraine has rebuffed a heavy-assault by Russian forces in eastern Ukraine.

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 in the face of dogged Ukrainian defense.

White House National Security Council spokesman John Kirby said the U.S. estimate is based on newly declassified American intelligence. He did not detail how the intelligence community derived the number.

Troops from Russia’s Wagner mercenary group and other forces are fighting Ukrainian troops house-to-house to try to gain control of what has become known as the “road of life” — the last remaining road west still in Ukrainian hands, which makes it critical for supplies and fresh troops. Both sides have cited gains in recent days.

Col. Gen. Oleksandr Syrskyi, the head of Ukrainian ground forces, said that Russia continued to exert “maximum effort” to take the city 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.