Finland's president and prime minister said on Thursday they were in favour of joining NATO and a formal decision would be taken this weekend, after Russia's war in Ukraine sparked a swift U-turn in opinion.
The Kremlin immediately responded to the announcement, saying it would have to take "military-technical" steps if Finland joined the Western military alliance, which would "definitely" pose a threat to Russia.
"The expansion of NATO and the approach of the alliance to our borders does not make the world and our continent more stable and secure," Kremlin spokesman Dmitry Peskov told reporters.
Several NATO members, including the US, Germany, Norway, Denmark and Poland, welcomed Finland's move.
Neighbouring Sweden, which like Finland has been militarily non-aligned for decades, is also expected to announce its decision on NATO membership in the coming days.
That decision will very likely come at a meeting on Sunday of Prime Minister Magdalena Andersson's Social Democratic Party.
The two countries are widely expected to submit their membership bids in unison.
"Finland must apply for NATO membership without delay," President Sauli Niinisto and Prime Minister Sanna Marin said in a joint statement.
"NATO membership would strengthen Finland's security. As a member of NATO, Finland would strengthen the entire defence alliance," the statement said.
A special committee will announce Helsinki's formal decision on a membership bid on Sunday, it added.
In Washington, Pentagon spokesman John Kirby said Vladimir Putin "didn't want a strong NATO on his western flank. He's getting that", adding that the Swedish and Finnish militaries "may be small, but they are powerful".
Moscow has repeatedly warned Stockholm and Helsinki of consequences if they were to join the alliance.
"Joining NATO would not be against anyone," Niinisto, who has often served as a mediator between Russia and the West, told reporters on Wednesday.
To Russia, he would say: "You caused this. Look in the mirror," he said.
In a blog post later Thursday, Niinisto struck a conciliatory note, writing: "Russia is and will remain Finland's border neighbour. It must continue to be able to handle practical matters with it."
As recently as January, amid tensions between the West and Russia, Marin said a NATO bid would be "very unlikely" during her current mandate, which ends in April 2023.
- Rattled by war -
But after its powerful eastern neighbour invaded Ukraine on February 24, Finland's political and public opinion swung dramatically in favour of membership as a deterrent against Russian aggression.
A poll published on Monday by public broadcaster Yle showed that a record 76 percent of Finns now support joining the alliance, up from the steady 20-30 percent registered in recent years.
A country of 5.5 million people, Finland shares a 1,300-kilometre (800-mile) border with Russia.
In 1939, it was invaded by the Soviet Union.
Finns put up a fierce fight during the Winter War but were ultimately forced to cede a huge stretch of its eastern Karelia province in a peace treaty with Moscow.
Defence Minister Antti Kaikkonen said Thursday on his blog he hoped Sweden would come to the same conclusion and "we could apply for membership together."
Swedish Foreign Minister Ann Linde said "Finland's decision is naturally of great importance to Sweden" and said her government would announce its decision "soon".
- 'Swift' process to join -
There has been broad political support for NATO membership in Finland, amid a general view that Russia's invasion has eroded the security situation in Europe.
Finns questioned by AFP after Thursday's announcement were also unanimous in their support.
"We stood alone in 1939, we don't want to stand alone again," said Nick Paterson, a 56-year-old entrepreneur.
"It's very important that the leaders of our country are on the same page and I think it was a good decision," said Ville Laakso, a 31-year-old lawyer.
Finland and Sweden have long cooperated with NATO, and are expected to be able to join the alliance quickly.
NATO Secretary General Jens Stoltenberg said Thursday Finland's entry would be "smooth and swift".
Finland's next step is a meeting on Sunday of a special committee made up of the president, prime minister and up to six other cabinet ministers. It will make the formal decision whether to submit a Finnish application.
The proposal will then be presented to parliament for a debate on Monday.
After an official bid is submitted to the alliance, negotiations get underway. Lawmakers in all 30 NATO member states then need to ratify Finland's application, a process that can take up to a year.
Foreign Minister Pekka Haavisto said on Tuesday he believed Finland could be a full NATO member "at the earliest" on October 1.