Brussels, Belgium - Finland is officially a member of NATO after Finnish Foreign Minister Pekka Haavisto deposited accession papers with the US on Tuesday in Brussels.

Finland becomes the 31st member of the alliance after overcoming opposition from Turkey. This is a "historic day," NATO Secretary General Jens Stotlenberg said.

Article 5 of the NATO treaty, a powerful collective security guarantee, now applies to Finland. An attack on one member of the alliance is an attack on all allies.

The Finnish flag is set to be hoisted in front of NATO headquarters, alphabetically placed between the flags of Estonia and France. Finland then participates for the first time in a NATO foreign ministers' meeting as a full-fledged member.

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Finland applied together with Sweden for NATO membership in May 2022 in the aftermath of the Russian invasion of Ukraine, but its application got tangled up in Turkey's opposition to Sweden's entry.

All NATO members must unanimously agree to admit new members. After months of talks brokered by the alliance, Turkey eventually relented and voted last week to admit Finland into NATO.

The decision ends Finland's decades-long neutral status amid perceived security risks due to Russia's full-scale invasion of Ukraine.

The Nordic country shares a 832-mile-long border with Russia.

NATO completed the final formalities to admit Finland after Turkey deposited ratification papers with US Secretary of State Antony Blinken.

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After Turkey deposited the membership ratification with Washington, Stoltenberg issued an invitation letter to Finland to join the alliance.

Helsinki lodged its acceptance of NATO's invitation to join the alliance with the US and completed the process.

Moscow reacted with fury. "The expansion of NATO is an attack on our security and Russia's national interests," Kremlin spokesman Dmitry Peskov said, according to the Interfax news agency on Tuesday.

Russia would be forced to take countermeasures accordingly, he said.

Sweden's membership is still pending Turkish ratification due to a number of sticking points, among them Ankara's concerns over what it says is a lack of cooperation in fighting terrorism.

Among other things, Turkey continues to block Swedish accession on the grounds that Sweden refuses to extradite 120 people viewed by Ankara as terrorists. Hungary has also not yet ratified Sweden's NATO membership.

In what he described as Finland's first act as a member of NATO, Haavisto gave his country's ratification papers of Sweden's NATO membership to Blinken at the end of the ceremony.