President Putin announced he would station nuclear weapons in neighbouring Belarus on Saturday
Nato has condemned Russia's "dangerous" and "irresponsible" rhetoric after Vladimir Putin's decision to station tactical nuclear weapons in Belarus.
The organisation is "closely monitoring" the situation and said the move would not lead it to change its own nuclear strategy.
The US said it did not believe Russia was preparing to use nuclear weapons.
Belarus shares a long border with Ukraine, as well as with Nato members Poland, Lithuania and Latvia.
Ukraine has called for an emergency meeting of the UN Security Council to address the potential threat of President Putin's announcement on Saturday.
President Putin said Moscow would not be transferring control of its arms to Minsk and that Belarusian leader Alexander Lukashenko - a firm ally of the Kremlin and supporter of its invasion of Ukraine - had long raised the issue with him.
Ukraine says the move violates nuclear non-proliferation agreements - an accusation President Putin has denied, instead comparing it to the US stationing its weapons in Europe.
But Nato on Sunday described Russia's reference to nuclear sharing as "misleading".
"Nato allies act with full respect of their international commitments," Nato spokesperson Oana Lungescu said.
The military alliance also accused Russia of consistently breaking its own arms control commitments, including the country's decision to suspend the new START treaty - a deal signed in 2010 which limits the number of US and Russian nuclear warheads and gives each the power to inspect the other's weapons.
EU foreign policy chief Josep Borrell urged Belarus to opt out of the deal with Putin, warning the country could face further sanctions if it went through with it.
"Belarus can still stop it, it is their choice," he wrote on Twitter.
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On Sunday, a top security adviser to Ukrainian President Volodymyr Zelensky accused Russia of making Belarus a "nuclear hostage".
Oleksiy Danilov wrote on Twitter that Russia's plans were a "step towards internal destabilisation" in Belarus and predicted anti-Russian sentiment in the country would grow.
Russia will start training crews to operate the weapons from next week. The construction of a storage facility for tactical nuclear weapons in Belarus will be completed by 1 July, President Putin said.
The announcement comes only days after Chinese President Xi Jinping's visit to Moscow, during which Russia and China issued a joint statement saying "all nuclear powers must not deploy their nuclear weapons beyond their national territories, and they must withdraw all nuclear weapons deployed abroad".