Russia has summoned the Finnish ambassador in Moscow, after Finland signed a new agreement on military co-operation with the US.
Monday's deal grants the US broad access to the area of Finland's long border with Russia.
Moscow said it would "take necessary measures to counter the aggressive decisions of Finland and its Nato allies".
Finland joined Nato this year in response to Russia's Ukraine invasion.
Russian President Vladimir Putin recently accused Nato of having "dragged" Finland into the bloc, and announced the creation of a new military district near Finland's border.
Meanwhile Finland has accused Russia of channelling migrants towards its territory in a "hybrid operation", and has temporarily closed all its border crossings with its eastern neighbour.
The agreement signed on Monday allows the US rapid military access and aid to Finland in the event of conflict, as well as joint training of forces.
"We do not expect the United States to take care of the defence of Finland. We continue to invest in our defence and share the burden in our area and beyond," Defence Minister Antti Hakkanen said, quoted by the AFP news agency.
"However, this agreement significantly enhances our ability to act together in all situations."
Russian foreign ministry spokeswoman Maria Zakharova said Moscow would not leave "Nato's military buildup" on its borders unanswered.
"Responsibility for turning a zone of good-neighbourliness in this region into a zone of possible confrontation lies entirely with the current Finnish authorities," she said.
Finland enjoyed relatively friendly relations with Russia's predecessor the USSR during the Cold War, steering clear of joining the Western military alliance.
But along with Sweden it applied to join Nato over concerns that Russia's aggression in Ukraine could spread to other neighbouring countries.
It shares a 1,340km (830-mile) frontier with Russia.
