There were no new comments from Russia on delivery of 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.

The US said it will "monitor" the movements, calling them "irresponsible and provocative".

"We have not seen any reason to adjust our own nuclear posture... nor any indication that Russia is prepared to use nuclear weapons from Belarus," White House Press Secretary Karine Jean-Pierre said.

In March, Putin announced he would station tactical -- or short-range -- nuclear weapons on 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 it would change the course of the conflict.

Belarusian troops began training on nuclear-capable Russian missile systems in April.