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. 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.