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.
Mr Putin said transferring the tactical nuclear warheads would be completed by the end of the summer.