William Burns said on Thursday that the counterattacks in the south and around Kharkiv in the north east showed how Vladimir Putin had understimated Ukraine’s courage.
“[It is] hard to see Putin’s record in the war as anything but a failure,” Mr Burns said, as reported by the New York Times.
“Putin’s bet right now is that he is going to be tougher than the Ukrainians, the Europeans, the Americans....
“I believe, and my colleagues at CIA believe, that Putin is as wrong about that bet as he was profoundly wrong in his assumptions going back to last February about Ukrainian will to resist.”
Mr Burns, speaking at the Billington CyberSecurity conference in Washington, implied the US and European allies must continue to stand their ground.
Countries have imposed sanctions on Russia and sought other sources of energy to weaken the Kremlin’s economic position.
This week, the state-owned Gazprom has released a dystopian video of Europe suffering the effects of a winter without their gas.
“Not only has the weakness of the Russian military been exposed,” Mr Burns added.
“But there is going to be long-term damage done to the Russian economy and to generations of Russians.”