The Kremlin vowed on Tuesday to take "countermeasures" after Finland's accession to NATO, describing the expansion of the alliance as "undermining the security" of Russia .

Kremlin spokesman Dmitry Peskov told reporters, "This is a new escalation of the situation. The expansion of NATO constitutes a violation of our security and our national interests."

"This forces us to take countermeasures. We will closely follow what is happening in Finland (...) how this matter poses a threat to us. Relevant measures will be taken. Our army will present (developments) the situation at the appropriate time," he added.

After adopting a policy of military non-alignment for three decades, Finland joined NATO on Tuesday in a strategic shift that was caused by the Russian attack on Ukraine.

Finland's accession allows the alliance to double the length of its borders, which its members share with Russia, which raises Moscow's displeasure.

Russia sees the US-led alliance as one of the main threats to its security. Kiev's desire to join the alliance was one of the reasons cited by Moscow to justify its military attack on Ukraine.

"Finland has never become anti-Russian and there is no disagreement" with it, Peskov said, explaining that its accession to the alliance "can only affect the nature of our relations" because the alliance "is not a friendly organization, but rather hostile to Russia on more than one level."

For its part, the Russian Foreign Ministry confirmed that the "countermeasures" that Moscow will take "will depend in particular on the deployment of NATO weapons or not and their quality on Finnish territory."

"Concrete measures related to the defense of Russia's northwestern borders depend on the concrete conditions for (Finland's) accession to NATO, especially the deployment of NATO's military infrastructure and weapon systems capable of launching raids on its territory," the ministry added in a statement.

On Tuesday, Finland joined NATO, becoming the 31st member of this system, after a military non-alignment policy it adopted for three decades, which constitutes a strategic turning point for the alliance that angered Russia.

And the Russian Foreign Ministry considered that by joining NATO, Finland had abandoned its "identity" and "any independence."

According to the foreign ministry, "Finland has become one of the small member states of the alliance that decides nothing and loses its voice in international affairs."