The announcement comes after Russia on Tuesday branded Finland's NATO membership an "assault on our security" and said it would take countermeasures.

In this context, Putin on March 25 said he would station tactical nuclear arms on Russia's ally, a move that drew widespread criticism.

Tactical nuclear arms are battlefield weapons that, while devastating, have a smaller yield compared to long-range strategic weapons.

"An Iskander-M operational-tactical missile system has been transferred to Belarusian forces," Defence Minister Sergei Shoigu said on Tuesday.

"On April 3, Belarusian crews began training on how to use it," he said.

Belarus' defence ministry published photos of men in uniforms with the Belarusian crest, their faces hidden, getting on military planes to Russia.

"The crew of these units will study in detail issues relating to the maintenance and use of the tactical nuclear weapons of the Iskander-M missile system," the Belarusian defence ministry said on Telegram.

"They will undergo a full training cycle at one of the Russian armed forces' training grounds," it said, without saying how long the training would last.

Putin's announcement spurred fears of nuclear conflict, but experts and governments said it was unlikely that the move would change the course of the conflict.

Russia is building a special storage facility for such weapons, which it plans to finish on July 1.

Belarus's authoritarian leader Alexander Lukashenko said Friday he was ready to host "strategic" Russian nuclear weapons "if need be".