Belarusian leader Alexander Lukashenko said Thursday that Russia had begun moving nuclear weapons to its western neighbour and ally, after Russian President Vladimir Putin announced plans to transfer them in March.
There were no new comments from Russia on bringing the nuclear weapons to Belarus.
"The transfer of nuclear munitions has begun," Lukashenko told reporters during a visit to Moscow.
Lukashenko has allowed his territory — which borders Ukraine as well as EU and NATO members Poland and Lithuania — to serve as a launching pad for Russia's Ukraine offensive.
In March, Putin announced he would station tactical — or short-range — nuclear weapons to the territory of Belarus, drawing condemnation from the West.
Lukashenko said Putin had informed him on Wednesday that he had signed a decree on the transfer.
Belarusian opposition leader Svetlana Tikhanovskaya on Thursday said the move would " not only endanger the lives of Belarusians but also create a new threat against Ukraine and all of Europe."
"It will make Belarusians hostages to Russian imperial ambitions," Tikhanovskaya added.
Tactical nuclear arms are battlefield weapons that, while devastating, have a smaller yield compared to long-range strategic weapons.
Putin's announcement had spurred fears of nuclear conflict, but experts and governments said it was unlikely that the move would change the course of the conflict.
Belarusian troops began training on nuclear-capable Russian missile systems in April.