Kyiv on Sunday said Russia took Minsk as a "nuclear hostage" after President Vladimir Putin announced the deployment of tactical nuclear weapons to Belarus, bringing the arms to a country at the gates of the European Union.
Putin, who has issued thinly veiled warnings that Russia could use nuclear weapons if threatened, said the move was similar to the United States transferring weapons to allies.
"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".
On Saturday, Putin announced Russia would station tactical nuclear weapons to neighbor and ally Belarus "without violating our international agreements on nuclear non-proliferation".
Ukrainian presidential aide Mykhaylo Podolyak on Sunday accused the Russian leader of breaching such obligations.
"(Putin) admits that he is afraid of losing and all he can do is scare" people, Podolyak also said on Twitter.
Strongman Alexander Lukashenko, who has been in power in Belarus for almost 30 years, is a key Putin ally.
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".
In an interview broadcasted Saturday, Putin said the move to deploy tactical nuclear weapons to Belarus was "nothing unusual".
"The United States has been doing this for decades. They have long placed their tactical nuclear weapons on the territory of their allies," Putin said.
Putin said he spoke to Lukashenko and said "we agreed to do the same."
Russia will start training crews on April 3 and plans to finish the construction of a special storage facility for tactical nuclear weapons by July 1.
Putin has previously said nuclear tensions were "rising" globally but that Moscow would not deploy first.
The Russian leader said renewed discussions with Lukashenko on the issue were spurred by a British official's suggestion to send depleted uranium weapons to Ukraine.
Russia will respond if the West supplied Ukraine with such ammunition, he added.
"Russia of course has what it needs to answer. Without exaggeration, we have hundreds of thousands of such shells. We have not used them yet."
He said the weapons "can be classified as the most harmful and hazardous for humans... and also for the environment".
Depleted uranium munitions are highly effective at piercing armor plate, but their use is controversial.
The metal is toxic for the soldiers who use the weapons and for civilians in areas where they are fired.