London: One of Russian President Vladimir Putin's closest allies warned NATO on Thursday that if Sweden and Finland joined the US-led military alliance then Russia would deploy nuclear weapons and hypersonic missiles in a European exclave.

Finland, which shares a 1,300-km (810-mile) border with Russia, and Sweden are considering joining the NATO alliance. Finland will decide in the next few weeks, Prime Minister Sanna Marin said on Wednesday.

Dmitry Medvedev, deputy chairman of Russia's Security Council, said that should Sweden and Finland join NATO then Russia would have to strengthen its land, naval and air forces in the Baltic Sea.

Medvedev also explicitly raised the nuclear threat by saying that there could be no more talk of a "nuclear free" Baltic - where Russia has its Kaliningrad exclave sandwiched between Poland and Lithuania.

"There can be no more talk of any nuclear–free status for the Baltic - the balance must be restored," said Medvedev, who was Russian president from 2008 to 2012.

Medvedev said he hoped Finland and Sweden would see sense. If not, he said, they would have to live with nuclear weapons and hypersonic missiles close to home.

When asked how Washington views the potential addition of Sweden and Finland to NATO in light of Russia's warning, the US State Department said there was no change in Washington's position and repeated that "NATO's open door is an open door."

"Without speaking to any countries in particular, we would not be concerned that the expansion of a defensive alliance would do anything other than promote stability on the European continent," Department spokesperson Ned Price said in a briefing.

Russia has the world's biggest arsenal of nuclear warheads and along with China and the United States is one of the global leaders in hypersonic missile technology.

Lithuania said Russia's threats were nothing new and that Moscow had deployed nuclear weapons to Kaliningrad long before the war in Ukraine. NATO did not immediately respond to Russia's warning.

Still, the possible accession of Finland and Sweden into NATO - founded in 1949 to provide Western security against the Soviet Union - would be one of the biggest strategic consequences of the war in Ukraine.

Finland gained independence from Russia in 1917 and fought two wars against it during World War Two during which it lost some territory. On Thursday, Finland announced a military exercise in western Finland with the participation of Britain, the United States, Latvia and Estonia.

Sweden has not fought a war for 200 years. Foreign policy has focused on supporting democracy and nuclear disarmament.

Kaliningrad

Kaliningrad, formerly the port of Koenigsberg, capital of East Prussia, lies less than 1,400 km from London and Paris and 500 km from Berlin.

Russia said in 2018 it had deployed Iskander missiles to Kaliningrad, which was captured by the Red Army in April 1945 and ceded to the Soviet Union at the Potsdam conference.

The Iskander, known as SS-26 Stone by NATO, is a short-range tactical ballistic missile system that can carry nuclear warheads. Its official range is 500 km but some Western military sources suspect it may be much greater.

"No sane person wants higher prices and higher taxes, increased tensions along borders, Iskanders, hypersonics and ships with nuclear weapons literally at arm's length from their own home," Medvedev said.

"Let's hope that the common sense of our northern neighbours will win."

While Putin is Russia's paramount leader, Medvedev's comments reflect Kremlin thinking and he is a senior member of the security council - one of Putin's main chambers for decision making on strategic issues.

Lithuanian Defence Minister Arvydas Anusauskas said Russia had deployed nuclear weapons in Kaliningrad even before the war.

"Nuclear weapons have always been kept in Kaliningrad ... the international community, the countries in the region, are perfectly aware of this," Anusauskas was quoted as saying by BNS. "They use it as a threat."

Russia's Feb. 24 invasion of Ukraine has killed thousands of people, displaced millions and raised fears of a wider confrontation between Russia and the United States - by far the world's two biggest nuclear powers.

Putin says the "special military operation" in Ukraine is necessary because the United States was using Ukraine to threaten Russia and Moscow had to defend against the persecution of Russian-speaking people.

Ukraine says it is fighting an imperial-style land grab and that Putin's claims of genocide are nonsense. US President Joe Biden says Putin is a war criminal and a dictator.

Putin says the conflict in Ukraine as part of a much broader confrontation with the United States which he says is trying to enforce its hegemony even as its dominance over the international order declines.