Russia has vowed to "strengthen its military potential" along its border with Finland as a direct response to Finland’s accension to the NATO ranks on Tuesday.
Russia’s Deputy Foreign Minister Alexander Grushko was quoted by Russian news site RIA Novosti as saying the expansion of NATO called for "additional steps" in military security.
“In the event that forces and assets of other NATO members are deployed in Finland, we will take additional steps to reliably ensure Russia's military security," Mr Grushko said.
"We will strengthen our military potential in the western and north-western direction.”
Russian Defence Minister Sergei Shoigu said last year that Russia was taking "adequate countermeasures" and would form 12 units and divisions in its western military district.
The border between Finland and Russia is the longest Russia shares with a European country — some 1,340km long.
In February, Finland began constructing a fence along the divide due to a rise in Russians fleeing conscription to the war in Ukraine.
Finland is set to join NATO on Tuesday in a step that NATO chief Jens Stoltenberg said will make Finland safer and the collective alliance stronger.
"We will raise the Finnish flag for the first time here at NATO headquarters. It will be a good day for Finland's security, for Nordic security and for NATO as a whole.”
Finland will become the sixth NATO nation to share a border with Russia, joining Norway, Latvia, Estonia, Lithuania and Poland.
Alarmed by Russia's invasion of Ukraine a year ago, Finland abandoned its decades-long policy of non-alignment and applied to join the alliance.
Joining the alliance was hampered by a refusal to ratify the application by Hungary and Türkiye, both of whom relented last week.
The Finnish contribution adds more than 257,000 troops to NATO's combined forces.
ABC/ Reuters