"We will fight until the end, we are not going to live in a Russia-managed state." Korsunsky told Reuters in an interview on Friday.
"There is no reason to believe that he (Putin) can threaten us, as he thinks, to the state that we will overthrow our government and we will embrace Russia after what they have done to us. No way. Therefore he may use nuclear weapons."
Late last month, Putin ordered his military command to put Russia's deterrence forces - which include nuclear arms - on high alert, citing what he called aggressive statements by NATO leaders and Western economic sanctions against Moscow.
Putin has not directly threatened to use nuclear arms. But while announcing the military operation in Ukraine last month, he said after referring to Russia's powerful nuclear arsenal: "Whoever tries to hinder us ... should know that Russia's response will be immediate. And it will lead you to such consequences that you have never encountered in your history."
Korsunsky said that Ukraine was relying on its allies to help avoid any kind of escalation.
"We try to work with our partners, major nuclear powers who know how to monitor the situation on nuclear weapons, and which can help us not to allow this to happen," he said.
Moscow calls its actions in Ukraine a "special military operation. It says it wants to "demilitarise" and "denazify" its pro-Western neighbour and prevent Kyiv from joining NATO.Â
