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 Russian statement (in Russian) described Finland's move as "a radical change in the country's foreign policy".
"Finland's accession to Nato will cause serious damage to bilateral Russian-Finnish relations and the maintaining of stability and security in the Northern European region," it said.
"Russia will be forced to take retaliatory steps, both of a military-technical and other nature, in order to neutralise the threats to its national security that arise from this."
However, Moscow has not specified what steps it plans to take.
"Everything will depend on how this expansion process plays out, the extent to which military infrastructure moves closer to our borders," Kremlin spokesman Dmitry Peskov earlier told reporters.
