Russia has denounced as "a dangerous historic mistake" Finland's membership in the NATO, saying the move would weaken security in Northern Europe and increase the risk of conflict.
"The security of these Nordic countries will not increase, but rather decline as a result of Finland's accession to NATO," Russian Deputy Foreign Minister Sergei Ryabkov said on Tuesday.
He also warned that Moscow would take compensatory measures in the military-technical sphere, stressing that any threats from the northern flank toward Russia would be fended off.
The senior Russian diplomat further noted that Finland and Sweden had fallen victim to "the Russophobic frenzy," adding that they had failed to put their interests above those of the collective West.
Finland and Sweden applied last year to join NATO following Russia's invasion of Ukraine. All 30 allies signed Finland's and Sweden's accession protocols. Turkey and Hungary delayed the process for months but recently agreed on Finland. Ankara has sought guarantees and assurances from the two Nordic states, notably on tackling what it calls extremism. Hungary's demands have not been explicit.
Finland officially joined NATO on Tuesday despite repeated warnings from Russia.
Russia's reaction to the new membership was strong.
Russia's Foreign Ministry condemned the move in a statement, saying Finland was committing a dangerous historic mistake by joining NATO and that the move would dilute the country's own influence on the world stage and hurt its ties with Moscow.
"Helsinki's policy of military non-alignment had long served Finnish national interests and was an important factor of confidence-building in the Baltic Sea region and the European continent as a whole," the ministry said.
"This is now a thing of the past. Finland has become one of the small members of the alliance that doesn't decide anything, losing its special voice in international affairs. We are sure that history will judge this hasty step," it added.
Russian Defense Minister Sergei Shoigu also warned that Finland's NATO membership "creates the risks of a significant expansion of conflict," but it would not affect the outcome of Russia's "special military operation" in Ukraine.
Kremlin spokesman Dmitry Peskov also said that Russia would be forced to take "counter-measures" in response to Finland's accession to NATO, depending on the kind of military deployments and infrastructure rolled out in Finland.
The further expansion of NATO is an "encroachment on our security and on Russia's national interests," Peskov said.
Finland shares a 1,340-kilometer eastern frontier with Russia, whose border with NATO has now doubled. Russia has vowed to bolster its military capacity in its western and northwestern regions, near the border with the Nordic country, if the alliance deploys any additional troops or equipment to its new member.