Russian casualties in Ukraine have already reached around 15,000 killed and perhaps 45,000 wounded, CIA Director William Burns said, adding that Ukraine has also endured significant casualties.

“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. So a quite significant set of losses,” Burns said.

“And, the Ukrainians have suffered as well – probably a little less than that. But, you know, significant casualties.” Burns added.

Earlier in March, Russia admitted that it had lost 1351 personnel during the military operations but has not provided any official numbers since.

Burns made these remarks was speaking at the Aspen Security Forum in Colorado on Wednesday (Jul 20).

Burns also scotched speculation that the Russian president Vladimir Putin is suffering from serious illness as he pursues his war on Ukraine.

"There are lots of rumours about President Putin's health and as far as we can tell he's entirely too healthy," Burns said at the Aspen Security Forum in Colorado.

Burns, who has previously served as ambassador to Moscow, said he had been observing and dealing with the Russian leader for over two decades.

"He is convinced that his destiny as Russia's leader is to restore Russia as a great power. He believes the key to doing that is to recreate a sphere of influence in Russia's neighbourhood and he cannot do that without controlling Ukraine." Burns said during the discussion.

Burns suggested that Russia's current concentration of forces in the Donbas points to the Russian military learning hard lessons.

Burns termed the invasion of Ukraine as a "strategic failure" for Putin because he had hoped to topple the Kyiv government within a week.

"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," Burns said.

Burns also said that the Russian invasion of Ukraine is influencing China's strategic calculations regarding a potential attack on Taiwan.

"I suspect the lesson that the Chinese leadership and military are drawing is that you've got to amass overwhelming force if you're going to contemplate that in the future," Burns said.

"Our sense is that it probably affects less the question of whether the Chinese leadership might choose some years down the road to use force to control Taiwan, but how and when they would do it," Burns said.