Russia has already stationed a first batch of tactical nuclear weapons in Belarus, Vladimir Putin has said.
Speaking at an economic forum, he said they would only be used if Russian territory or statehood was threatened.
The US government says there is no indication that the Kremlin plans to use nuclear weapons to attack Ukraine.
“We see no indication that Russia is preparing to use a nuclear weapon,” US Secretary of State Antony Blinken said after Putin’s remarks.
Belarus is a key Russian ally and served as the springboard for Mr Putin’s full-scale invasion of Ukraine in February last year.
Tactical nuclear warheads are less powerful than most of the rest of the Russian nuclear arsenal. Mr Putin said the transfer would be completed by the end of the summer.
Responding to questions after a speech at the St Petersburg International Economic Forum, the Russian president said it was about “containment” and a reminder to anyone “thinking about inflicting a strategic defeat on us”.
Asked by the forum moderator about the possibility of using these weapons, he replied: “Why should we threaten the whole world? I have already said that the use of extreme measures is possible in case of danger to the Russian state.”
The Russian leader is due to meet African leaders in St Petersburg after their visit to Kyiv on Friday as part of a peace initiative they are presenting to the two countries.
However, while in the city, they were attacked by Russian missiles.
South African President Cyril Ramaphosa has called for de-escalation on both sides and negotiations for peace.
“We came here to listen and recognize what the Ukrainian people have been through,” he said.
But Ukrainian President Volodymyr Zelensky has said that instead of making diplomatic overtures to Russia, it should be diplomatically frozen for sending a message that the international community condemns its invasion.
kyiv will not enter into negotiations with Moscow as long as it still occupies Ukrainian territory, Zelensky said.
Mr. Putin also reiterated that Ukraine had no chance of succeeding in its ongoing counter-offensive.
Ukraine’s military also lacked its own military equipment and would soon only use equipment provided by the West, he said.
“You can’t fight for long like this,” he said, warning that any US F16 fighter given to Ukraine “will burn, there’s no doubt about it.”
Ukraine has previously dismissed similar remarks, saying it was making progress in reclaiming territory in eastern and southern Ukraine.
The BBC cannot independently verify battlefield claims.
The Russian leader also touched on economic themes, saying Western sanctions against Russia had failed to isolate it and had instead led to an expansion of its trade with “the markets of the future”.
He hailed the new agreements with countries in Asia, the Middle East and Latin America, calling them “reliable and responsible partners”.