Russian President Vladimir Putin said Saturday he would deploy tactical nuclear weapons in neighbour and ally Belarus. Ukraine on Sunday called for an emergency meeting of the United Nation's Security Council, accusing Moscow of holding Minsk as a "nuclear hostage".
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 Belarusian leader Alexander Lukashenko and said "we agreed to do the same."
Putin has previously issued thinly veiled warnings that he could use nuclear weapons in Ukraine if Russia were threatened, reviving Cold War-era fears.
The move saw Ukraine's foreign ministry call on the United Nations to hold an emergency meeting.
"Ukraine expects effective actions to counteract the Kremlin's nuclear blackmail from the United Kingdom, China, the United States and France... We demand that an extraordinary meeting of the UN Security Council be immediately convened for this purpose," the Ukrainian foreign ministry said on Sunday.
Earlier, Ukraine's National Security and Defence Council secretary Oleksiy Danilov, condemned the move on Twitter.
"The Kremlin took Belarus as a nuclear hostage," he said, adding it was "a step towards the internal destabilization of the country".
For Danilov, Putin's announcement "maximizes the level of negative perception and public rejection of Russia and Putin in Belarusian society".
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 his country's territory.
Fears have since risen that Belarus may join its ally's offensive, but Lukashenko said he would do so "only if attacked".
Russia has also helped equip Belarusian planes "without violating our international agreements on nuclear-non-proliferation... 10 planes are ready for this type of weapon to be used," Putin said.
Russia has given Belarus an Iskander system that can carry nuclear weapons, he indicated.
It will start training crews on 3 April and plans to finish the construction of a special storage facility for tactical nuclear weapons by 1 July.
Depleted uranium ammunition
Putin also said that Russia would respond if the West supplied Ukraine with depleted uranium ammunition.
"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," Putin added in an interview on Russian television.
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 armour 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.
Putin has previously said nuclear tensions were "rising" globally but said Moscow would not deploy first.
The International Campaign to Abolish Nuclear Weapons (ICAN) has warned nuclear threats were creating a dangerous sense of uncertainty around their possible use.
The longer Russia's operation in Ukraine grinds on, the greater the nuclear strike risk, ICAN warned last month ahead of the offensive's first anniversary.
Fears of a 'dirty bomb'
Putin announced last month that Moscow would suspend its participation in New START, the last remaining arms control treaty between the world's two main nuclear powers, Russia and the United States.
NATO chief Jens Stoltenberg slammed the move, saying it marked the end of Europe's post-Cold War arms control architecture.
The announcement came after Moscow last August suspended US inspections of its military sites under New START.
US officials have voiced fears that Russia could use nuclear weapons if it feels routed on the battlefield and could plant a fictitious story to justify its actions.
Russia has already spoken of supposed Ukrainian attempts to detonate a "dirty bomb," drawing strong denials from Ukraine and a sharp rebuke from the United States, which had rare direct communication with Moscow to warn against nuclear use.
Neither the United States nor Russia -- by far the largest nuclear weapons powers -- officially has a policy of no first use of the ultra-destructive arms.