Finland must seek immediate membership in NATO, leaders of the Nordic nation said on Thursday, moving to end seven decades outside the Western military bloc in response to Russia’s war in Ukraine.

Finland’s entry would add significant combat power to the alliance while also deepening the East-West divisions that have consumed Europe since Russia’s Feb. 24 invasion of Ukraine. Neighboring Sweden is expected to announce its own NATO bid soon.

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Moscow said that Finnish accession, which would add hundreds of miles to NATO’s shared border with Russia, would threaten its security. Kremlin spokesman Dmitri Peksov said that Finnish membership could require new measures by Russia to “balance the situation.”

Finnish President Sauli Niinisto and Prime Minister Sanna Marin, announcing their positions after weeks of internal deliberations, said the militarily nonaligned nation must “apply for NATO membership without delay.”

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“As a member of NATO, Finland would strengthen the entire defense alliance,” they said in a statement. The decision, which must be approved by Finland’s parliament, is expected to be finalized in coming days.

Finnish Foreign Minister Pekka Haavisto said President Vladimir Putin’s assault on Ukraine had altered the security situation not only for Finland. “The war started by Russia jeopardizes the security and stability of the whole of Europe,” he told European lawmakers.

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It was not immediately clear what steps NATO nations might take to protect Finland and Sweden from any Russia retaliation until they are formally brought under NATO’s mutual defense umbrella, a process that Western officials have said could be completed by the time leaders of the alliance gather in Spain in late June.

British Prime Minister Boris Johnson on Wednesday announced new measures to bolster Finland and Sweden’s security, including greater intelligence sharing and joint training.

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NATO nations are increasing the flow of weapons and other aid to leaders in Kyiv, in an accelerating effort to help Ukraine fend off Russia’s brutal assault. At the same time, the United States and its allies have led the effort to impose punishing sanctions on Russia, bringing Moscow’s relations with the West to their worst point since the Cold War.

Putin has long cited NATO’s eastward expansion — from its founding group of 12 nations in 1949, all from Western Europe and North America, to its 30 members today, including a clutch of former Soviet and Warsaw Pact states — as a major threat to Russian security.

“The expansion of NATO does not make our continent more stable and secure,” Peskov told journalists Thursday, according to Russian news outlet Interfax. “NATO is moving in our direction,” he said.

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Russia’s Foreign Ministry characterized the decision as “a radical change” in Finland’s foreign policy, saying it contravened a non-alignment stance that — according to the ministry — has served Moscow and Helsinki well.

Dmitry Medvedev, the deputy chairman of Russia’s Security Council and a former Russian president, said that NATO’s support of Ukraine, along with military exercises in countries bordering Russia, “increase the likelihood of a direct and open conflict.”

“This kind of conflict is always at risk of turning into a full-fledged nuclear war,” Medvedev said.

The foreign ministry said Russia would be “forced to take retaliatory steps, both of a military-technical and other nature.”

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In addition to fueling support in Nordic nations for joining NATO, Russia’s invasion has also driven countries in the former Soviet sphere closer to the West. Both Ukraine and Moldova are now actively seeking membership in the European Union.

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NATO Secretary General Jens Stoltenberg predicted that a Finnish accession process “would be smooth and swift,” according to Reuters. “Finland is one of NATO’s closest partners, a mature democracy, a member of the European Union, and an important contributor to Euro-Atlantic security,” he said.

In the capitals of the European Union and other NATO countries, the Finnish leaders’ statement was greeted with expressions of support and promises to keep the application process as short as possible.

Swedish Foreign Affairs Minister Ann Linde on Thursday said her country should “take [the Finnish] assessments into account” when making its own decision on NATO membership. The Swedish tabloid Expressen reported that a decision from Sweden on joining the defense alliance could come as soon as Monday, citing unnamed sources.

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Now that Finnish leaders have expressed their support for a NATO membership bid, the Ministerial Committee on Foreign and Security Policy will meet with Finland’s president to formally decide whether the country should apply, then submit a proposal to lawmakers. The committee is set to meet Sunday, Agence France Press reported, citing Finnish newspaper Iltalehti.

The Finnish Parliament’s defense committee has already recommended joining NATO, while the major parliamentary parties have also expressed support for a military alliance. Li Andersson, chair of the Left Alliance in Finland’s Parliament, which has been plagued by internal disagreements over the prospect of NATO membership, wrote that she was prepared to support it.

Jaclyn Peiser, Andrew Jeong and Kim Bellware contributed to this report.