The most senior US general estimates that around 100,000 Russian and 100,000 Ukrainian soldiers have been killed or injured in the war in Ukraine.
Gen Mark Milley, chairman of the US Joint Chiefs of Staff, also suggested that around 40,000 civilians had died after being caught up in the conflict.
The estimates are the highest offered yet by a Western official.
But he observed that signs Kyiv was willing to re-enter talks with Moscow offered “a window” for negotiations.
In recent days, Ukraine has signalled a willingness to hold some discussions with Moscow, after President Volodymyr Zelensky dropped a demand that his opposite number, Vladimir Putin, must be removed from power before negotiations could resume.
But speaking in New York, Gen Milley added that for any talks to be successful, both Russia and Ukraine would have to reach a “mutual recognition” that a wartime victory “is maybe not achievable through military means, and therefore you need to turn to other means”.
The top general – who serves as President Joe Biden’s most senior military adviser – said the scale of the casualties could convince both Moscow and Kyiv of the need to negotiate over the coming winter months, when fighting may slow due to freezing conditions.
On Wednesday, Moscow announced its troops would start to withdraw from the key southern city of Kherson – the only major city to fall to Russian forces.
Gen Milley said that while “initial indicators” suggested that a retreat had begun, he observed that Russia had amassed around 20-30,000 troops in the city, and the withdrawal could take several weeks.
“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.
The news of Russia’s withdrawal from Kherson comes after Russian President Vladimir Putin called up some 300,000 reservists to fight in Ukraine in September.
Military experts in the West and Ukraine say the need for mobilisation showed that Russian troops were failing badly on the battlefield in Ukraine.