Russia has said there will be “no more talk of a nuclear–free Baltic” if Sweden and Finland join NATO.
Such a development would more than double the length of the military alliance’s land borders with Russia, Moscow added.
Sweden and Finland are reviewing their security arrangements following Vladimir Putin’s invasion of Ukraine.
Polls in both countries have shown support for joining NATO.
Dmitry Medvedev says Moscow could have ‘more officially registered opponents’. File pic
But the deputy chairman of Russia’s Security Council, Dmitry Medvedev, said Moscow would respond, were that to happen.
“There can be no more talk of any nuclear-free status for the Baltic – the balance must be restored,” he said.
“Until today, Russia has not taken such measures and was not going to.”
Referring to land borders, Mr Medvedev said: “Naturally (they) will have to be strengthened.”
He added that Russia would “seriously strengthen the grouping of ground forces and air defence (and) deploy significant naval forces in the Gulf of Finland”.
In response, Lithuanian Defence Minister Arvydas Anusauskas said Russia already has nuclear weapons in the Baltic region.
They have been deployed in Russia’s Kaliningrad enclave on the Baltic Sea since before the invasion of Ukraine began, Mr Anusauskas told the BNS news agency.
Kaliningrad, on the shore of the Baltic Sea, is sandwiched between NATO members Lithuania and Poland.
“The current Russian threats look quite strange when we know that, even without the present security situation, they keep the weapon 100 km from Lithuania’s border,” the minister said.
“Nuclear weapons have always been kept in Kaliningrad. The international community, the countries in the region, are perfectly aware of this. They use it as a threat.”
When Russia “threatens” it is “nothing new”, Lithuanian prime minister Ingrida Simonyte told reporters.
Russian officers take the camouflage off a Topol-12M mobile nuclear missile. File pic
Mr Medvedev said Moscow would have “more officially registered opponents” if NATO admitted extra members.
Writing on Telegram, he claimed NATO was preparing to admit Finland and Sweden with “minimal bureaucratic procedures”.
Russia’s response should be considered with “no emotion, with a cold head”, he continued.
The Swedish and Finnish prime ministers, Magdalena Andersson and Sanna Marin, took part in a joint press conference in Stockholm on Wednesday.
Ms Marin said Finland was ready to make a decision on joining NATO “within weeks” following an extensive debate in the 200-seat Eduskunta legislature.
Mr Medvedev claimed Swedish and Finnish opinion on joining the alliance was “split in half” despite the “maximum efforts of home-grown propagandists”.
He also denied that the invasion of Ukraine had led to the consideration of membership.
“Attempts to drag them into the alliance have been made before,” he said.