Russia could launch attacks across NATO's northernmost member states in an offensive involving some 600,000 troops, alliance sources fear.
Satellite images have emerged showing a build up of Russian forces just miles from the Finnish border, with evidence that Moscow has been establishing troop accommodation, aircraft infrastructure and other new facilities at key military bases.
Signs that Moscow is sending weapons and troops to the area come following claims, including from Finland's Prime Minister and German intelligence, that Vladimir Putin is gearing up for a lengthy conflict with the West.
A Finnish government report cited in news outlet Iltalehti in December states that Helsinki considers an attack on Finland, Norway, Sweden and the Baltic states to be a possibility.
'Russia is strengthening its military presence and activities in its northwestern direction in all operational environments as quickly as possible,' the report warns.
