Russia has said Finland and Sweden's entry into Nato would lead to a militarisation of the Baltic region.
Finland and Sweden are both expected to make a formal request to join the Western alliance, following Russia's invasion of Ukraine.
Russian President Vladimir Putin warned his Finnish counterpart Sauli Niinistö that joining Nato and abandoning Finland's neutral status would be a "mistake."
Putin also said there was no threat to Finland's security, with which Russia shares a 1,300-km (810-mile) border.
The Russian foreign ministry has indicated there will be retaliation, and the country's decision to suspend electricity supplies to Finland is being seen as an early sign of this.