"The Kremlin took Belarus as a nuclear hostage," the secretary of Ukraine's National Security and Defence Council, Oleksiy Danilov, wrote on Twitter.

He added that the move was "a step towards the internal destabilization of the country".

Strongman Alexander Lukashenko, who has been in power in Belarus for almost 30 years, is a key Putin ally.

On Saturday, Putin said he and Lukashenko "agreed" Russia would station tactical nuclear weapons in Belarus "without violating our international agreements on nuclear non-proliferation".

On Sunday, Ukrainian presidential aide Mykhaylo Podolyak accused the Russian leader of "violating the nuclear non-proliferation treaty".

"(Putin) admits that he is afraid of losing and all he can do is scare" people, Podolyak also said on Twitter.

Back in February 2022, Minsk allowed the Kremlin to launch its invasion of Ukraine from Belarusian territory.

Fears have since risen that Belarus may join its ally's offensive, but Lukashenko said he would do so "only if attacked".

For Danilov, Putin's announcement "maximizes the level of negative perception and public rejection of Russia and Putin in Belarusian society".