Moscow's all-out invasion of Ukraine last year upended Europe's security landscape and prompted Finland, and its neighbor Sweden, to drop decades of military non-alignment.

"Not so many years ago we thought it was unthinkable that Finland would become a member. Now they will be a fully-fledged member of our alliance and that is truly historic," NATO Secretary-General Jens Stoltenberg said.

"We are removing the room for miscalculation in Moscow about NATO's readiness to protect Finland, and that makes Finland safer."

Finnish Defence Minister Antti Kaikkonen called it "a win-win situation" ahead of the choreographed final formalities before Finland's blue-and-white flag can be hoisted in front of NATO's headquarters. 

But Moscow decried the move as an "assault" on Russia's security and national interests.

"This forces us to take countermeasures [...] in tactical and strategic terms," Kremlin spokesman Dmitry Peskov said. 

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