Russia's foreign ministry on Friday warned of unspecified "consequences" should Finland and Sweden join the US-led NATO defence alliance, after Moscow sent troops into Ukraine.

Moscow's military actions in Ukraine have sparked a dramatic U-turn in public and political opinion in both Finland and Sweden over long-held policies of military non-alignment.

Finland said this week it will decide whether to apply for NATO membership within weeks, and Sweden is also discussing joining the alliance.

"The choice is up to the authorities of Sweden and Finland. But they should understand the consequences of such a step for our bilateral relations and for the architecture of European security as a whole," Russian foreign ministry spokeswoman Maria Zakharova said in a statement.

She added that Sweden and Finland's membership in NATO is "unlikely to contribute to strengthening (their) international prestige". 

Zakharova said that the non-alignment policy of the two countries "provided a reliable level of security", while membership in a military alliance is "not capable of strengthening their national security". 

"They will automatically find themselves on the NATO frontline," Zakharova said.

Former Russian president Dmitry Medvedev warned Thursday that Russia would deploy nuclear weapons close to the three Baltic states and Scandinavia if Finland or Sweden decided to join.