Russia will have to bolster its defences on the border if Finland joins NATO, its ambassador to the EU has warned.
Speaking to Sky News' Beth Rigby, Vladimir Chizhov said it would "necessitate certain military technical measures, like raising or improving the degree of defence preparations along the Finnish border".
Finland's leaders earlier said it must apply to join the military alliance "without delay" to "strengthen" its security.
The country shares a vast border of more than 800 miles with its much larger neighbour - which invaded it at the start of the Second World War.
Mr Chizhov's warning echoes the Kremlin's earlier comments that Finland joining NATO would "definitely" be a threat to Russia and would trigger "retaliatory steps".
The ambassador said such a move would "certainly necessitate rethinking of Russian defence posture" but wouldn't "necessarily [involve] troops and tanks, but certain preparations definitely... like radars, perhaps".
Russia has long sought to prevent NATO from expanding near its borders and wants Ukraine to drop any ambition to join.
Finland's president and prime minister made a joint statement on its membership ambitions earlier on Thursday.
They said they hoped a decision on applying would be "taken rapidly within the next few days".
"NATO membership would strengthen Finland's security. As a member of NATO, Finland would strengthen the entire defence alliance," the statement added
"Finland must apply for NATO membership without delay."
NATO chief Jens Stoltenberg said Finland would be welcomed and that the accession would be "smooth and swift".
Neighbouring Sweden is expected to decide on joining NATO in the coming days.
Russia's EU ambassador also insisted the war in Ukraine - or its "special military operation" as it calls it - "was proceeding according to schedule" but was "not as quick as some people wold have wanted".
Efforts to capture Kyiv failed and led the country to instead focus on capturing the eastern Donbas region.
However, analysts say it is still making slow progress and Ukraine has claimed to have won back some areas.
"Had Russian army followed the pattern of US and other Western countries, we would have steamrolled Ukraine several times by now. But that was never the intention," Mr Chizhov told Sky News.