Illustration shows Finnish and NATO flags
Finnish and NATO flags are seen printed on paper this illustration taken April 13, 2022. Reuters

Russia said on Tuesday Finland's accession to the NATO military alliance was a dangerous historic mistake that would weaken security in the wider region, increase the risk of conflict, and force Moscow to take counter-measures.

Finland formally joined NATO on Tuesday, its flag unfurling outside the military bloc's Brussels headquarters, in a historic policy shift away from non-alignment that was brought about by Russia's invasion of Ukraine.

In Moscow, pro-Kremlin activists held a noisy anti-NATO demonstration outside the U.S. Embassy, holding up placards with slogans such as "Stop NATO" and "NATO is a sponsor of Nazism."

Russia and Finland share a 1,300-km (800-mile) border, and Moscow has already said it will beef up military divisions stationed in its west and northwest.

Defence Minister Sergei Shoigu told Russia's military leadership that Finland's accession "creates the risks of a significant expansion of conflict".

But he said it would not affect the outcome of what Russia calls its "special military operation" in Ukraine.

One of the main reasons Russia says it is fighting in Ukraine is to stop its neighbour being turned into a NATO outpost that would threaten its own security.

The Kremlin said Russia would be forced to take "counter-measures" in response to Finland's NATO entry that it would adjust depending on the kind of military deployments and infrastructure rolled out in Finland.

The further expansion of NATO is an "encroachment on our security and on Russia's national interests", said Kremlin spokesman Dmitry Peskov.

Russia's Foreign Ministry said Finland was committing a dangerous historic mistake by joining NATO that would dilute its own influence on the world stage and hurt its ties with Moscow.

"Helsinki's policy of military non-alignment had long served Finnish national interests and was an important factor of confidence-building in the Baltic Sea region and the European continent as a whole," the ministry said in a statement.

"This is now a thing of the past. Finland has become one of the small members of the alliance that doesn't decide anything, losing its special voice in international affairs. We are sure that history will judge this hasty step."

In St Petersburg, only about 150 km (93 miles) from the Finnish border, a resident who gave his name as Nikolai told Reuters that Russians would now view Finland as a hostile state.

"There is absolutely no doubt that Finland is making problems for itself by joining (NATO) because we used to consider it a brotherly country of the capitalist world, the closest to us in spirit, in relations, in mutually beneficial economic relations," he said.

"But now we will consider it as a state that is unfriendly to us."