Former Russian president Dmitry Medvedev, a close ally of Vladimir Putin, has warned if Finland and Sweden decide to join Nato, Russia will deploy nuclear weapons to the Baltic States and Scandinavia.
In a message on Telegram, Medvedev, deputy chairman of Russia's security council and president from 2008 to 2012, said if Sweden and Finland drop their neutral status and join Nato, this would more than double Russia's land border with Nato members.
"Naturally, we will have to reinforce these borders," he said.
"In this case, it would not be possible to talk any more about the Baltic non-nuclear status. The balance has to be restored," he added.
Medvedev said Russia would "seriously reinforce its group of ground forces and air defences and deploy significant naval forces in the Gulf of Finland".
The threat has been confirmed by Kremlin spokesman Dmitry Peskov, who said "this has been talked about many times" and Putin had issued an order on "reinforcing our western flank" because of Nato's growing military potential.
When asked if the reinforcement would include nuclear weapons, Peskov said he "couldn't say".
"There will be a whole list of measures, necessary steps. This will be covered at a separate meeting by the president," he said.
Finland and Sweden have hinted at a dramatic U-turn in their long-held policies of military non-alignment, after the invasion of Ukraine by Russia.
Earlier this week, Finland said it will decide whether to apply for Nato membership within weeks.
Sweden is also said to be discussing a membership application.
Both countries have remained neutral in recent decades to avoid risking their relationship with Russia.
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During the Cold War, Finland agreed to stay out of western security cooperation in exchange for guarantees from Moscow not to invade the country.
However, the invasion of Ukraine has led to a shift in how both countries look at their policies and their geopolitical status.
Footage emerged earlier this week showing Russian missiles heading along a road towards Helsinki.
While the video's authenticity has not yet been verified, it could suggest Russia is bolstering defences on its border with Finland, much like it did with Ukraine before the invasion.
