Russia said Monday it would boost its military presence in the west and northwest of the country in response to Finland joining NATO.
Secretary General Jens Stoltenberg announced earlier in the day that Finland will become the 31st member of the military alliance on Tuesday.
"We will strengthen our military potential in the western and northwestern direction," Russian Deputy Foreign Minister Alexander Grushko was quoted as saying by state news agency RIA Novosti.
"In the event of the deployment of forces and resources of other NATO members on the territory of Finland, we will take additional steps to reliably ensure Russia's military security," he added.
Russia and Finland share a 1,300-kilometre (800-mile) border.
Finland and Sweden dropped a decades-long policy of military non-alignment and applied to join the western alliance last May in the wake of Moscow's offensive in Ukraine.
The Kremlin insisted in March that Russia was not a threat to the two Nordic countries and has no "dispute" with them.
Russia, however, sees NATO enlargement as an "existential" threat to its security and has used Ukraine's wish to join the alliance to justify the offensive.