Finland formally became the 31st member state of the NATO, ending its military non-alignment in the fastest accession procedure in record time. Meanwhile, Russia said it could be forced to take "counter-measures".
The national flag of Finland was raised and the country's anthem was played for the first time at NATO headquarters on Tuesday afternoon in a brief ceremony.
"The era of military non-alignment in our history has come to an end. A new era begins," Finland's President Sauli Niinisto said at the ceremony on Tuesday, adding that he hoped to see neighbouring Sweden join soon.
"Finland's membership is not complete without that of Sweden."
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"This is a historic day. We welcome Finland as the newest member of our Alliance," NATO Secretary General Jens Stoltenberg said.
Finland received its formal invitation to join the Alliance from US Secretary of State Antony Blinken earlier on Tuesday. In exchange, Finland's Foreign Minister Pekka Haavisto handed over his country's instruments of accession, thus completing formalities, Xinhua news agency reported.
As Finland did not stop investing in defence after the end of the Cold War, the country is bringing well-trained and well-equipped military forces with a high level of readiness, Stoltenberg added.
Finland's NATO accession will not change the country's foreign policy, Haavisto said during Tuesday's ceremony.
"Finland's membership isn't targeted against anyone ... Finland is a stable and predictable Nordic country that seeks peaceful resolution of disputes," he added.
Finland and Sweden both applied for NATO membership in May last year and were formally invited to join the Alliance in June.
The accession process requires the approval of all member states. Turkey was the last NATO member to ratify Finland's accession bid, while Sweden's path to the Alliance remains blocked by Turkey and Hungary.
Regarding Sweden's accession process, Stoltenberg said that Finland, Turkey and Sweden will continue trilateral meetings on Stockholm's bid.
Finland's formal NATO accession elicited an immediate response from Russia. Kremlin spokesman Dmitry Peskov told a press briefing on Tuesday that Russia considered it another aggravation of the situation. In Russia's opinion, the expansion of NATO infringes on its security and national interests.
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The expansion of NATO "forces Russia to take counter-measures to ensure its own security," Peskov was quoted by Russia's state-owned Sputnik News Agency as saying.
Commenting on Finland's NATO accession, Russia's Foreign Ministry said it was an "ill-conceived step" that would negatively affect relations between Helsinki and Moscow.
