Finland became the 31st member of the NATO military alliance Tuesday in a historic policy shift brought on by Russia's invasion of Ukraine, drawing a threat from Moscow of "countermeasures."

The Nordic country got its blessing from Türkiye, the last NATO member to approve its membership after the Turkish Parliament ratified it last week.

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.

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.

NATO Secretary-General Jens Stoltenberg, welcoming Finland to its ranks, noted that Russian President Vladimir Putin had cited opposing the expansion of the alliance 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.

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

Russia said 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 Zelenskiyy's chief of staff, Andriy Yermak, wrote on Telegram: "FI made the right choice. NATO is also a key goal for Ukraine."

The event marks the end of an era of military nonalignment for Finland that began after the country repelled an invasion attempt by the Soviet Union during World War II and opted to try to maintain friendly relations with neighboring Russia.

But the invasion of Ukraine in February 2022 prompted Finns to seek security under NATO's collective defense pact, which states that an attack on one member is an attack on all.

Moscow, which has long criticized the move, reacted crossly.

Meanwhile, Kremlin spokesperson Dmitry Peskov said the NATO expansion was an "encroachment on our security and Russia's national interests." He said Moscow would watch closely for any NATO military deployments in Finland.

"I'm tempted to say this is maybe the one thing we can thank (Mr.) Putin for. Because he once again here precipitated something he claims to want to prevent," Blinken said.

Sweden and Finland applied together last year to join NATO, but the Swedish application has been held up by NATO members Türkiye and Hungary.

However, Swedish Foreign Minister Tobias Billstroem told reporters it was Stockholm's ambition to become a member at the NATO summit in Vilnius in July.

"This is a question of utmost importance for Sweden ... There is no reason for neither the Turkish nor the Hungarian Parliament to make any further delays," he said.

Türkiye says Stockholm harbors members of terrorist groups, namely PKK and Gülenist Terror Group (FETÖ), and has demanded their extradition to ratify Swedish membership.

Foreign Minister Mevlüt Çavuşoğlu said "effective cooperation" against terrorism is "a must" with Sweden for the country's NATO bid.

Çavuşoğlu's remarks came after his meeting with Swedish counterpart Tobias Billstrom on the sidelines of a two-day NATO foreign ministers meeting in Brussels on Tuesday.

"Reiterated our expectations for Sweden's NATO accession to Foreign Minister Tobias Billstrom," Cavusoglu said in a tweet. "Effective cooperation against terrorism is a must."

He also attached a photo of himself with Billstrom, as well as one from the meeting between the two sides.

US, UK urge Türkiye, Hungary to let Sweden join NATO

U.S. Secretary of State Antony Blinken marked Finland's arrival in the NATO alliance on Tuesday with an appeal to Hungary and Türkiye to drop their objections to Sweden following suit.

"Sweden is also a strong and capable partner that is ready to join NATO," Blinken said.

"We encourage Turkey and Hungary to ratify the accession protocols for Sweden without delay so we can welcome Sweden into the alliance as soon as possible."

President Joe Biden said he is "proud" to welcome Finland into NATO, expanding the Western military alliance in direct response to Russia's brutal invasion of neighboring pro-Western Ukraine.

"Together — strengthened by our newest ally Finland — we will continue to preserve transatlantic security, defend every inch of NATO territory," Biden said in a statement. He also called on Türkiye and Hungary to join the rest of NATO "without delay" in ratifying Sweden's entry into the alliance.

British Prime Minister Rishi Sunak also hailed Finland's "historic" accession to NATO and urged the military grouping to admit Sweden next, after its entry was blocked by Hungary and Türkiye.

Finland's formal accession as the 31st member of NATO "has made our Alliance stronger and every one of us safer", Sunak said.

"All NATO members now need to take the steps necessary to admit Sweden too, so we can stand together as one Alliance to defend freedom in Europe and across the world."

Foreign Secretary James Cleverly, attending the NATO meeting in Brussels where Finland acceded, announced another £12 million ($16 million) in "non-lethal" military aid to Ukraine such as medical equipment and rations.

"Russia thought its aggression would divide us. Instead, we are bound tighter together, resolute in our defence of the principles of freedom and the rule of law," Cleverly said.

"Let us be clear that our door remains open. We will welcome further allies with open arms and we continue to push for Sweden's swift accession."

German Chancellor Olaf Scholz also welcomed Finland's accession to NATO on Tuesday and offered his support to Sweden's efforts to join the alliance.

Finland's NATO entry was "good news and a win for transatlantic security", Scholz said on Twitter. "Sweden's still pending accession has our full support," he added.