Army Gen. Mark Milley said Wednesday that the U.S. believes both Russia and Ukraine have suffered casualties significantly higher than what has been reported.

"You're looking at well over 100,000 Russian soldiers killed and wounded," Gen Milley, also the chairman of the Joint Chiefs of Staff, said. "Same thing probably on the Ukrainian side."

Milley was speaking at the Economic Club of New York on Wednesday when he acknowledged there could be a respite in fighting as a harsh winter looms over the upcoming months.

Army Gen. Mark Milley spoke about the ongoing war in Ukraine while at the Economic Club of New York on Wednesday.

In response to reports of Russia's withdrawal from the Ukrainian city of Kherson, Milley said, "The initial indicators are they are in fact doing it. They made the public announcement they're doing it."

Ukraine remains skeptical of the withdrawal, as President Volodymyr Zelenskyy has warned that the Russians could be feigning a pullout from Kherson to lure the Ukrainian army into an ambush.

Milley noted that it could take weeks for Russia to withdraw its troops, adding that the move could be an effort "to preserve their force to reestablish defensive lines south of the [Dnieper] river, but that remains to be seen."

In his remarks Milley did leave an opening for the chance of diplomacy, saying "there's also an opportunity here, a window of opportunity for negotiation."

"When peace can be achieved, seize it," he said. "Seize the moment."

Milley, President Biden's top military adviser, also said "mutual recognition" is critical for sides to begin negotiations. He said victory in this case "is maybe not achievable through military means, and therefore you need to turn to other means," citing the brutal end of World War I as an example.

In August, a New York Times report said U.S. officials estimated that 70,000 to 80,000 Russians had been killed or wounded, using "satellite imagery, communication intercepts, social media and on-the-ground media reports to arrive at that figure."