Russia has lost an estimated 15,000 troops so far in its war against Ukraine, CIA Director William Burns said Wednesday.

Speaking at the Aspen Security Forum in Colorado, Burns said he believed Russian President Vladimir Putin was wrong in his assumption that he could fracture the NATO alliance before the war began.

- I think the latest estimates from the US intelligence community would be something in the vicinity of 15,000 killed and maybe three times that wounded. So a quite significant set of losses - he said.

He also noted that Ukrainians have suffered as well in the ongoing war. Burns also said the Russian leader faces an alliance that is just about to add Finland and Sweden. Putin thought he could establish his dominance over Ukraine very quickly, he added.

Over 5,100 civilians have been killed in Ukraine since the beginning of the war on Feb. 24 this year, according to UN figures.

At least 16 million people in Ukraine have also been forced to flee their homes, including around 9.2 million that have fled to other countries.