"The Kremlin believes that this is the latest aggravation of the situation," Kremlin spokesman Dmitry Peskov told reporters.
"The expansion of NATO is an assault on our security and Russia's national interests," he added.
"And this forces us to take countermeasures... in tactical and strategic terms." He did not provide further details.
Finland becomes the 31st member of NATO on Tuesday, in a historic strategic shift provoked by Moscow's assault on Ukraine, which doubles the US-led alliance's border with Russia.
The start last year of the Kremlin's offensive in Ukraine upended Europe's security landscape and prompted Finland -- and its neighbour Sweden -- to drop decades of military non-alignment.
In a separate statement, the Russian foreign ministry said that by joining NATO Finland had lost "its self-identity and any independence."
The enlargement of the US-led alliance's border with Russia has "profoundly changed the situation" in northern Europe, the statement said.
The foreign ministry said Russia's further steps would depend on "concrete conditions" of Finland's NATO integration including "the deployment of NATO military infrastructure and strike weapons systems on its territory."
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