Russia has said it will be forced to take “retaliatory steps” over its neighbour Finland’s move to join Nato.
A foreign ministry statement said the move would seriously damage bilateral relations, as well as security and stability in northern Europe.
Earlier, Finland’s president and PM called for the country to apply for Nato membership “without delay”.
It comes amid a surge in public support for Nato membership following Russia’s invasion of Ukraine.
Finland shares a 1,300-km (810-mile) border with Russia. Until now, it has stayed out of Nato to avoid antagonising its eastern neighbour.
Finland will formally announce its decision on Sunday after it has been considered by parliament and other senior political figures.
Sweden has said it will announce a similar decision on the same day.
Nato Secretary General Jens Stoltenberg has said he expects the process of giving Sweden and Finland membership to happen “quite quickly”.
The White House said the US would back a Nato application from both countries if they apply.