"We are not convinced that Finland and Sweden joining NATO will somehow strengthen or improve the security architecture on our continent," Kremlin spokesman Dmitry Peskov told reporters.

"This is a serious issue, an issue that raises our concern, and we will follow very carefully what will be the consequences of this accession of Finland and Sweden to NATO in a practical sense in terms of our security, which must be ensured in an absolutely unconditional manner," he said.

But he also pointed out that in comparison with Ukraine, Russia did not have any territorial disputes with Finland or Sweden.

Earlier on Monday, Deputy Foreign Minister Sergei Ryabkov said the decision was a "grave mistake with far-reaching consequences."

Finland and Sweden are poised to jettison decades of military non-alignment to join NATO as a defence against feared aggression from Russia after Moscow sent troops into Ukraine on February 24.

Moscow has warned Finland, with which it shares a 1,300 kilometre (800 mile) border, that it would take "reciprocal steps".

Finnish President Sauli Niinisto spoke with Russian leader Vladimir Putin on Saturday about the country's application for NATO membership.

Finland announced its intention to join NATO on Sunday as Sweden's ruling party said it backed membership, paving the way for a joint application.