The head of the Central Intelligence Agency (CIA) has estimated Russian has lost some 15,000 soldiers since its invasion of Ukraine in February.
CIA director William Burns said that Ukraine had inflicted “significant losses” on Russian forces since the invasion began five months ago.
Another 45,000 Russian soldiers are also estimated to have been wounded during the invasion, Mr Burns added.
“The latest estimates from the US intelligence community would be something in the vicinity of 15,000 (Russian forces) killed and maybe three times that wounded,” he told the Aspen Security Forum in Colorado.
Describing the numbers as a “quite significant set of losses”, Mr Burns added that Ukraine had also suffered heavy casualties while fighting Russian forces.
He told the conference: “The Ukrainians have suffered as well – probably a little less than that. But, you know, significant casualties.”
Russia has not made public the actual figure of casualties in its war, with the only update on 25 March declaring that 1,351 of its soldiers had been killed one month into the invasion.
After failing to capture Kyiv and large swathes of the country at the beginning of the conflict, Russian forces now control about one fifth of Ukraine.
The country’s defence minister Sergei Shoigu said earlier this month his country had “full control” over the eastern region of Luhansk, which forms about half of Ukraine’s Donbas region.
Mr Burns said Russia had concentrated its forces there and could once again try extending its control beyond eastern Ukraine.
“In a way, what the Russian military has done is retreat to a more comfortable way of war, in a sense, by using their advantages and long range firepower to stand off and effectively destroy Ukrainian targets and to compensate for the weaknesses in manpower that they still experience,” Mr Burns said.
The United States has provided intelligence to help guide Ukraine its battlefield decisions, while Congress in May passed a $40bn package of funding for military hardware and humanitarian aid.
Additional reporting by Reuters