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 on Russia's state television late on Sunday, Lukashenko, President Vladimir Putin's staunchest ally among Russia's neighbors, 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.
Kazakh President Kassym-Jomart Tokayev, whose nation of 20 million people has close historical ties with Moscow but has refused to recognize Russia's annexation of parts of Ukraine, dismissed Lukashenko's invitation to join the union.
"I appreciated his joke," Tokayev's office quoted him as saying on Telegram, adding that Kazakhstan was already a member of a broader Russian-led trade bloc, the Eurasian Economic Union, so no further integration was necessary.
"As for nuclear weapons, we do not need them because we have joined the Nuclear Non-Proliferation Treaty and Comprehensive Nuclear-Test-Ban Treaty," he said in a remark which could be interpreted as a sting to Moscow and Minsk.
"We remain committed to our obligations under those international documents."
Russia used the territory of Belarus as a launchpad for its invasion of their common neighbor Ukraine in February last year, and since then their military cooperation has intensified, with joint training exercises on Belarusian soil.
On Sunday, the Belarusian Defense 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.