The two Nordic countries look set to renounce decades of military neutrality and join the defensive alliance, which has triggered outcry from Moscow.
The addition of both nations to Nato would represent Putin's worst nightmare - an expansion of the military alliance to his borders - and it would follow a series of battlefield setbacks in Ukraine.
He has tried to justify his invasion of Ukraine and destabilisation of eastern Europe as a counter to Nato's addition of ex-Soviet states and former Warsaw Pact members, demanding the alliance effectively withdraw to its position before the mid-1990s.
After Putin said Finland was making a mistake joining the alliance, Russian TV said: "Their official reason is fear. But they'll have more fear in Nato.
"When Nato bases appear in Sweden and Finland, Russia will have no choice but to neutralise the imbalance and new threat by deploying tactical nuclear weapons."
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