MOSCOW

Russian President Vladimir Putin warned Ukraine on Friday that any use of nuclear weapons against Russia would be Kyiv’s “last mistake.”

Speaking at the St. Petersburg International Economic Forum, Putin warned that if Ukraine tries to drop a so-called “dirty bomb” on Russian territory, it would trigger a "mirror response" from Moscow.

“This would be a colossal mistake by those we refer to as neo-Nazis operating in present-day Ukraine, perhaps their last,” he said.

Putin noted that Russia's nuclear doctrine provides for a stern response to existential threats, adding that the consequences for Kyiv would be dire.

“Our response would be extremely harsh, most likely catastrophic, both for the neo-Nazi regime and for Ukraine itself,” he said.

At the same time, Putin acknowledged that Russia currently has no confirmed evidence suggesting Ukraine plans to use a dirty bomb.

“Thank God, we have no proof of such intentions. But we operate under the assumption that such an idea might arise in someone’s sick mind,” he said.

Putin further said that Moscow is establishing a "security zone" along the border with Ukraine, in response to repeated attacks on Russian territory and a recent incursion into the Kursk region.

Putin said the depth of this buffer zone could range from 8 to 12 kilometers (5-7.5 miles), and noted that Russia could potentially capture the city of Sumy, a regional center in northeastern Ukraine.

“The depth is about 10 to 12 kilometers, sometimes 8, sometimes 12. Beyond that lies the city of Sumy. Capturing it is not our task, but I don’t rule it out,” he said.

When asked how far Russia intends to advance in Ukraine, Putin said he believed that the Russian and Ukrainian peoples are one nation and in this sense, "all of Ukraine is ours.”

“Where the foot of a Russian soldier steps – that is ours,” he said.