Russia said on Monday that the West should have no illusions that Moscow will simply put up with the Nordic expansion of the U.S.-led NATO military alliance to include Sweden and Finland, casting the move as a mistake that would stoke military tension.
Vladimir Putin, Russia's paramount leader since the last day of 1999, has repeatedly cited the post-Soviet enlargement of the NATO alliance eastwards towards Russia's borders as a reason for the invasion of Ukraine.
The war, though, has fomented one of the biggest changes to Europe's security architecture for decades: once unthinkable moves by Sweden and Finland, which shares a 1,300 km (800 mile) border with Russia, to join the military alliance.
"They should have no illusions that we will simply put up with it - and nor should Brussels, Washington and other NATO capitals," Russian Deputy Foreign Minister Sergei Ryabkov was quoted as saying by the state RIA news agency.
Ryabkov, who led talks with the United States on a doomed Russian proposal to halt NATO's eastward expansion, said the decisions by Helsinki and Stockholm to join the alliance were a mistake.
"The general level of military tension will rise, predictability in this sphere will decrease. It is a shame that common sense is being sacrificed to some phantom provision about what should be done in this unfolding situation," Ryabkov said.
Russia has given few clues about what it will do in response to the Nordic enlargement of NATO, saying merely that there would be a "military-technical response".
One of Putin's closest allies said last month that Russia could deploy nuclear weapons and hypersonic missiles in the Russian exclave of Kaliningrad if Finland and Sweden joined NATO.
The accession of Finland and Sweden into NATO - founded in 1949 to provide European security against the Soviet Union - would be one of the biggest strategic consequences of Russia's invasion of Ukraine to date.
The West says NATO - an alliance of 30 countries including former Warsaw Pact republics such as Poland and Hungary as well as nuclear powers the United States, Britain and France - is purely defensive.
Moscow says NATO threatens Russia and that Washington has repeatedly ignored the Kremlin's concerns about the security of its borders in the West, the source of two devastating European invasions in 1812 and 1941.
Finland gained independence from Russia in 1917 and fought two wars against it during World War Two during which it lost territory. Sweden has not fought a war for 200 years. Foreign policy has focused on supporting democracy and nuclear disarmament.
Putin says the "special military operation" in Ukraine is necessary because the United States was using Ukraine to threaten Russia through NATO enlargement and Moscow had to defend against the persecution of Russian-speaking people.
Putin says assurances were given as the Soviet Union collapsed that the alliance would not expand eastwards towards Russia, a promise he says was a lie that humiliated Russia in its time of historic weakness.
The United States and NATO dispute that such assurances were given explicitly. Kyiv and its Western backers say the claim of persecution of Russian speakers has been exaggerated by Moscow into a pretext for an unprovoked war against a sovereign state.
(Reporting by Reuters; editing by Guy Faulconbridge and Ed Osmond)