About 15,000 Russian troops have been killed and 45,000 wounded in the first six months of Moscow's war in Ukraine, CIA Director William Burns said on Wednesday.
The conflict is also taking a heavy toll on the Ukrainian military, he said, pointing to losses "probably a little less" than those of Russia.
"The latest estimates from the US intelligence community would be something in the vicinity of 15,000 [Russian troops] killed and maybe three times that many wounded,” Mr Burns told the Aspen Security Forum.
Ukrainian authorities said last month that up to 100 of their soldiers are killed every day in frontline fighting against Russian troops in the east.
"Significant casualties," Mr Burns said.
The CIA director, who was the last senior US official to meet Russian President Vladimir Putin before he ordered the February 24 invasion, described him as “relentlessly suspicious".
“He's professionally trained to be a cynic about human nature. He is relentlessly suspicious, always attuned to vulnerabilities that he can take advantage of," Mr Burns said.
The October meeting was designed as a US mission to dissuade Mr Putin from going to war in Ukraine, but Mr Burns said it was obvious then that the Russian leader was moving towards an invasion.
“The impression I conveyed back to the president [Joe Biden] when I got home, was that Putin hadn't yet made an irreversible decision to launch an invasion, but was clearly leaning hard in that direction,” Mr Burns said.
Now that Russia has failed to take Kyiv or overthrow the government in its "special military operation", Moscow has switched tactics and committed to a prolonged conflict, Mr Burns said.
Mr Putin's bet is that “he can succeed in a grinding war of attrition, that they can wear down the Ukrainian military … wear down European public and leadership and he can wear down the United States", he said.
But, he stressed: “Putin was wrong in his assumptions about breaking the alliance and breaking Ukrainian will before the war began, and I think he is just as wrong now.”
Asked about Mr Putin’s trip to Iran this week, Mr Burns said both countries need each other at the moment.
"Both are heavily sanctioned countries, both looking to break out of political isolation" he said.
But Mr Burns, a former ambassador to Moscow, said the Iranians and the Russians “don’t trust each other," describing them as "energy rivals and historical competitors".
“There are limits to the ways in which they're going to be able to help one another right now,” he said.