The admission of Finland and Sweden to NATO would be a "grave mistake with far-reaching consequences," says Russian Deputy Foreign Minister Sergei Ryabkov.
Military tensions would increase as a result, Ryabkov warned on Monday, according to the Interfax news agency. Addressing the two Scandinavian countries, he added that "they should have no illusions that we will simply resign ourselves to this."
Earlier, in a telephone conversation with Finnish head of state Sauli Niinistö, President Vladimir Putin spoke of "a mistake" by Helsinki, and insisted Russia posed no threat to Finland. The two countries share a 1300-kilometre border.
The plans for NATO membership are strongly influenced by Russia's invasion of Ukraine.
Finland, which has been neutral for decades, plans to apply soon for membership in the North Atlantic Treaty Organisation, which currently has 30 members.
Sweden, which is traditionally non-aligned, has also decided on an historic U-turn, with the governing Social Democrats now in favour of joining the Western military alliance.
The parliaments of both countries were to meet for debates on the issue on Monday.