Finland is mulling whether to join the NATO military alliance, Prime Minister Sanna Marin said, noting that a decision could come by next spring while citing a new “security environment” in Europe following Russia’s attack on Ukraine.

Speaking to fellow members of the ruling Social Democratic Party over the weekend, the PM said Moscow’s ongoing invasion is a “flagrant violation” of international law, arguing that “Russia is not the neighbor we thought it was.” She added that this has prompted the government to rethink its long-standing policy of neutrality toward the NATO bloc.

“In this new situation and changed security environment, we’ll have to evaluate all means to guarantee the safety of Finland and Finns,” Marin said. “We’ll have to seriously mull over our own stance and approach to military alignment. We’ll have to do this carefully but quickly, effectively during the course of this spring.”

Marin previously stated that NATO membership would be “very unlikely,” but has since changed her tune, now saying the assault on Ukraine had altered relations with Russia in “irreversible” ways. 

While Finland maintained a strict stance of non-alignment throughout the Cold War, refusing to enter NATO as well as the Soviet-led Warsaw Pact, recent polling indicates that Finns are increasingly favorable to membership in the North Atlantic alliance. A survey conducted in February showed a majority of respondents (53%) would back the idea, up from just 19% in 2017 – dubbed a “historic shift” by former Prime Minister Alex Stubb.

Opposition leaders such as Petteri Orpo, who heads up the National Coalition Party, have also publicly supported NATO membership for Finland, arguing that it is not only necessary for the country’s defense, but would mean it had fully joined the West. 

“In order to improve our security and guarantee our independence, we should join NATO. We still have a powerful and aggressive neighbor,” Orpo said, referring to Russia. “For me, NATO membership is not just about the pros and cons, it’s a bigger question of our identity. We are a western country… In this sense, our place is in NATO.”

However, some in government have objected to the idea, with one senior official telling the Financial Times the move may be seen as a dangerous provocation by Russia.

“Applying for membership carries awful risks for Finland, and for NATO,” they said. “I have always said that joining military alliances is something you do in quiet times, so that you do not import any instability into the organization you join.”

In comments to CNN on Sunday, NATO Secretary-General Jens Stoltenberg said that both Finland and Sweden would be “very much welcomed by all 30 allies,” even suggesting their membership could be fast-tracked.

Russia has long demanded an end to the eastward expansion of NATO, citing assurances from Washington that the alliance would not grow by “one inch” just before the fall of the Soviet Union. While it has vocally opposed membership for its neighbors Ukraine and Georgia, Moscow has also repeatedly warned that it would retaliate should Finland or Sweden join the bloc, though officials have yet to say how. 

“It is obvious that [if] Finland and Sweden join NATO, which is a military organization to begin with, there will be serious military and political consequences,” said Russian Foreign Ministry official Sergei Belyayev. “[It] would require changing the whole palette of relations with these countries and require retaliatory measures.”