Belarusian President Alexander Lukashenko said that if any other country wanted to join a Russia-Belarus union there could be "nuclear weapons for everyone."
Russia moved ahead last week with a plan to deploy tactical nuclear weapons in Belarus, in the Kremlin's first deployment of such warheads outside Russia since the 1991 fall of the Soviet Union, spurring concerns in the West.
In an interview published on Russia's state television late on Sunday, Lukashenko, President Vladimir Putin's staunchest ally among Russia's neighbours, said that it must be "strategically understood" that Minsk and Moscow have a unique chance to unite.
"No one is against Kazakhstan and other countries having the same close relations that we have with the Russian Federation," Lukashenko said.
"If someone is worried ... (then) it is very simple: join in the Union State of Belarus and Russia. That's all: there will be nuclear weapons for everyone."
He added that it was his own view - not the view of Russia.
Russia and Belarus are formally part of a Union State, a borderless union and alliance between the two former Soviet republics.
Russia used the territory of Belarus as a launchpad for its invasion of their common neighbour Ukraine in February last year, and since then their military cooperation has intensified, with joint training exercises on Belarusian soil.
On Sunday, the Belarusian Defence Ministry said that another unit of the S-400 mobile, surface-to-air missile systems arrived from Moscow, with the systems to be ready for combat duty soon. (Reporting by Lidia Kelly in Melbourne; editing by Guy Faulconbridge)