Finland's accession roughly doubles the length of the border that NATO shares with Russia and bolsters its eastern flank as the war in Ukraine grinds on with no resolution in sight.

Finnish Foreign Minister Pekka Haavisto completed the accession process by handing over an official document to U.S. Secretary of State Antony Blinken at NATO headquarters in Brussels where the Finnish flag was unfurled.

"We welcome Finland to the alliance," NATO Secretary General Jens Stoltenberg said at the end of the brief ceremony, according to Reuters.

Stoltenberg earlier noted that Russian President Vladimir Putin had cited opposing NATO's eastward enlargement as one justification for his invasion.

"He is getting exactly the opposite...Finland today, and soon also Sweden will become a full fledged member of the alliance," Stoltenberg said in Brussels.

"It is a great day for Finland and I want to say that it is an important day for NATO," Niinisto said at a joint news conference with Stoltenberg.

The Kremlin said Russia would be forced to take "counter-measures" to Finland's accession. Defence Minister Sergei Shoigu said the move raised the prospect of the conflict in Ukraine escalating further.

Russia had said on Monday it would strengthen its military capacity in its western and northwestern regions in response to Finland joining NATO.

The Ukrainian government also hailed Finland's move. President Volodymyr Zelenskiy's chief of staff Andriy Yermak wrote on Telegram: "FI made the right choice. NATO is also a key goal for Ukraine."

(With UNI inputs)