Russia has already stationed a first batch of tactical nuclear weapons in Belarus, Vladimir Putin says.
Speaking at an economic forum, he said they would only be used if Russia's territory or state was threatened.
The US government says there is no indication the Kremlin plans to use nuclear weapons to attack Ukraine.
"We don't see any indications that Russia is preparing to use a nuclear weapon," US Secretary of State Antony Blinken said after Mr Putin's comments.
Belarus is a key Russian ally and served as a launchpad for Mr Putin's full-scale invasion of Ukraine in February last year.
The tactical nuclear warheads are less powerful than much of the rest of Russia's nuclear arsenal. Mr Putin said the transfer would be finished by the end of the summer.
Answering questions after a speech at the St Petersburg International Economic Forum, Russia's president said the move was about "containment" and to remind anyone "thinking of inflicting a strategic defeat on us".
When asked by the the forum's moderator about the possibility of using those weapons, he replied: "Why should we threaten the whole world? I have already said that the use of extreme measures is possible in case there is a danger to Russian statehood."
The Russian leader is due to meet African leaders in St Petersburg after they visited Kyiv on Friday as part of a peace initiative they are presenting to both countries.
However while they were in the city it came under Russian missile attack.
South African President Cyril Ramaphosa called for de-escalation on both sides and negotiations for peace.
"We came here to listen and recognise what the people of Ukraine have gone through," he said.
But Ukraine's President Volodymyr Zelensky said instead of making diplomatic overtures to Russia it should be frozen out diplomatically to send a message that the international community condemned its invasion.
Kyiv would not enter negotiations with Moscow while it still occupied Ukrainian territory, Mr Zelensky said.
Mr Putin also repeated his claim that Ukraine stood no chance of succeeding in its ongoing counter-offensive.
The Ukrainian military was also running out of its own military equipment and would soon only be using Western-donated equipment, he said.
"You can't fight for long like that," he said, warning that any F16 US fighter jets given to Ukraine "will burn, no doubt about it".
Ukraine has previously dismissed similar remarks, asserting they are making progress in recapturing territory in both eastern and southern Ukraine.
The BBC cannot independently verify battlefield claims.
The Russian leader also addressed economic themes, claiming that Western sanctions on Russia had failed to isolate it and instead led to an "expansion" in its trade with "the markets of the future".
He praised new deals with countries in Asia, the Middle East and Latin America - calling them "reliable, responsible partners".