NATO Secretary-General Stoltenberg holds a news conference before NATO foreign ministers' meeting in Brussels
NATO Secretary-General Jens Stoltenberg attends a news conference before a meeting of NATO foreign ministers in Brussels, Belgium April 3, 2023. Reuters

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

Finland has a 1,300-km (810-mile) border with Russia, meaning NATO's frontier with Russia will roughly double in length, and the move drew a pledge from Moscow that it will beef up its forces in border regions.

"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".

"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 invasion of Ukraine in February last year pushed Finland and its neighbour Sweden to apply for NATO membership, abandoning decades of military non-alignment.

The last hurdle to Finland's membership was removed last week when Turkey'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."

Turkey continues to hold up Sweden's application. It says that Stockholm harbours 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 Turkey moving to do so.

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 finalised.

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