"The Russians have probably taken 70 or 80,000 casualties in less than six months," Under Secretary of Defense Colin Kahl said.

Kahl also said Russian forces have also lost "three or four thousand" armored vehicles, and could be running low on available precision-guided missiles, including air and sea-launched cruise missiles, after firing a large number on Ukraine targets since launching the invasion on February 24.

Those losses are "pretty remarkable considering the Russians have achieved none of Vladimir Putin's objectives at the beginning of the war," he told reporters, referring to the Russian president.

He said the slowdown in Russian forces' use of longer range and precision guided missiles was an indicator that their supplies had fallen close to what Moscow needed to hold in reserve for "other contingencies."

Kahl admitted that the Ukraine side also had significant losses of manpower on the battlefield, but gave no figures.

"Both sides are taking casualties. The war is the most intense conventional conflict in Europe since the Second World War," he said.

"But the Ukrainians have a lot of advantages, not the least of which is their will to fight."