Russia has already stationed a first batch of tactical nuclear weapons in Belarus, Vladimir Putin says.
Russia's president told a forum 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.
Mr Putin said transferring the tactical nuclear warheads would be completed 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 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."
