“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.
When asked whether Finland’s membership would be a threat, Peskov answered: “definitely”.
“Everything will depend on how this process takes place, how far the military infrastructure will move towards our borders,” he said.
Russia’s foreign ministry said Moscow would be “forced to take reciprocal steps, military-technical and other, to address the resulting threats to its national security”.
It accused NATO of seeking to create “another flank for the military threat to our country”.
“Helsinki should be aware of its responsibility and the consequences of such a move,” the foreign ministry said.
The president and prime minister of Finland stated their support for NATO membership earlier on Thursday, and a formal decision will be made this weekend.
After Russia sent troops into Ukraine on February 24, Finland’s political and public opinion dramatically shifted in favour of joining.
Finland shares an 800-mile (1,300-kilometer) border with Russia and has been nonaligned militarily for decades.