Russia will station tactical nuclear weapons in Belarus, President Vladimir Putin has said.
President Putin said the move would not violate nuclear non-proliferation agreements and compared it to the US stationing its weapons in Europe, according to Russian state media.
Moscow would not be transferring control of its arms to Minsk, he added.
The US said it did not believe Russia was preparing to use the nuclear weapons after the announcement.
"We have not seen any reason to adjust our own strategic nuclear posture," the US Defense Department said in a statement.
"We remain committed to the collective defence of the Nato alliance."
Belarus shares a long border Ukraine, and with Nato members Poland, Lithuania and Latvia.
This will be the first time since the mid-1990s that Moscow will have based nuclear arms outside the country.
The Soviet Union's collapse in 1991 meant weapons became based in four newly-independent states - Russia, Ukraine, Belarus and Kazakhstan - with the transfer of all warheads to Russia completed in 1996.
The Belarusian regime is a firm Kremlin ally and supporter of the invasion of Ukraine.
President Putin told Russian state television on Saturday that Belarusian leader Alexander Lukashenko had long raised the issue of stationing tactical nuclear weapons in Belarus.
"There is nothing unusual here either," he said. "Firstly, the United States has been doing this for decades. They have long deployed their tactical nuclear weapons on the territory of their allied countries."
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 added.
A small number of Iskander tactical missile systems, which can be used to launch nuclear weapons, have already been transferred to Belarus, President Putin said.
He did not specify when the weapons themselves would be sent.
The announcement to station weapons in Belarus 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."
On Sunday, a top security adviser to Ukrainian President Volodymyr Zelensky said that Russia's plans are a "step towards internal destabilisation" of Belarus and predicted anti-Russian sentiment in the country would grow. "The Kremlin took Belarus as a nuclear hostage," Oleksiy Danilov wrote on Twitter.
And exiled Belarusian opposition leader Svetlana Tikhanovskaya said that Russia's deployment of tactical nuclear weapons "grossly contradicts the will of the Belarusian people".
President Putin's comments come after President Zelensky renewed his plea for more military support from his Western allies.
Earlier this week, some 18 countries signed an agreement to supply the war-torn country with at least one million artillery shells over the next year.
But in an interview with a Japanese newspaper, President Zelensky said Ukraine could not launch a potential counter-offensive in the east of the country until further ammunition arrived.