Two hundred thousand soldiers have been killed in the nine months since Russia invaded Ukraine, a top US general has said.
Mark Milley, chairman of the Joint Chiefs of Staff, said as many as 40,000 Ukrainian civilians and "well over" 100,000 Russian soldiers have been killed or wounded in the war.
"Same thing probably on the Ukrainian side," Mr Milley added.
- UK to provide 1,000 extra surface-to-air missiles to Ukraine
- Russia orders troops to pull out from key Ukrainian city of Kherson
- Russia has lost twice as many aircraft in Ukraine than Afghanistan, Kyiv military chief says
"There has been a tremendous amount of suffering, human suffering," he said at The Economic Club of New York.
Ukrainian President Volodymyr Zelensky said on Tuesday he was open to peace talks with Russia to end the war but only on the condition that Russia returns all of Ukraine's occupied lands, provides compensation for war damage and faces prosecution for war crimes.
Russia has said it is open to talks and this week announced it had begun a retreat from Kherson.
Mr Zelensky has warned that the Russians are feigning a pullout from Kherson to lure the Ukrainian army into an entrenched battle in the strategic industrial port city, a gateway to the Russian-occupied Crimean Peninsula.
Mr Milley went on to say Russia had amassed 20,000 to 30,000 troops in Kherson and a full retreat could take several weeks.
"The initial indicators are they are in fact doing it. They made the public announcement they're doing it.
"I believe they're doing it in order to preserve their force to re-establish defensive lines south of the (Dnieper) river, but that remains to be seen," he said.
He said it is possible the Russians will use the retreat to reset their troops for a spring offensive but "there's also an opportunity here, a window of opportunity for negotiation".
But for negotiations to have a chance, both Russia and Ukraine would have to reach a "mutual recognition" that a military victory "is maybe not achievable through military means, and therefore you need to turn to other means", Mr Milley said, citing the end of the First World War as an example.