Finland will join NATO on Tuesday, marking the completion of a swift journey into the military alliance for the Nordic nation following Russia's offensive in Ukraine, officials said.

Finland has a 1,300-kilometer border with Russia, meaning NATO's frontier with Russia will roughly double in length. The move drew a pledge from Moscow that it will beef up its forces in border regions.

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"Tomorrow we will welcome Finland as the 31st member of NATO, making Finland safer and our alliance stronger," NATO Secretary-General Jens Stoltenberg told reporters in Brussels, hailing the move as "historic."

'A good day for NATO'

"We will raise the Finnish flag for the first time here at NATO headquarters. It will be a good day for Finland's security, for Nordic security and for NATO as a whole," Stoltenberg said.

Finnish President Sauli Niinisto will travel to Brussels to take part in the ceremony, his office said.

Russia's military campaign in Ukraine in February last year pushed Finland and its neighbor Sweden to apply for NATO membership, abandoning decades of military non-alignment. The last hurdle to Finland's membership was removed last week when Türkiye's parliament voted to ratify Helsinki's application even as it kept Sweden's bid on hold.

"President Putin went to war against Ukraine with the clear aim to get less NATO," Stoltenberg said. "He's getting the exact opposite."

Türkiye continues to hold up Sweden's application. It says that Stockholm harbors members of what Ankara considers terrorist groups – a charge Sweden denies – and has demanded their extradition as a step toward ratifying Swedish membership.

Hungary is also holding up Sweden's admission, citing grievances over criticism of Prime Minister Viktor Orban's policies. But NATO diplomats say they expect Budapest will approve Sweden's bid if it sees Türkiye moving to do so.

Russia's 'adequate countermeasures'

Stoltenberg pledged to work hard to get Sweden into NATO as soon as possible. He also stressed that NATO and Swedish officials were already working to bring Sweden closer to the alliance even as they wait for membership to be finalized.

In Moscow, Deputy Foreign Minister Alexander Grushko said Russia will boost its forces in its west and northwest.

Grushko told state-owned news agency RIA reported: "In the event that the forces and resources of other NATO members are deployed in Finland, we will take additional steps to reliably ensure Russia's military security."

Russian Defense Minister Sergei Shoigu said last year that Russia was taking "adequate countermeasures" and would form 12 units and divisions in its western military district.