Russian President Vladimir Putin warned his Finnish counterpart, Sauli Niinistö, that the Nordic country's renunciation of neutrality to join NATO would be a "wrong" decision.
"Putin stressed that renunciation of the traditional policy of military neutrality would be wrong, as there is no threat to Finland's security," the Kremlin said in a statement.
He stressed that such a change in Helsinki's foreign policy "may negatively influence Russian-Finnish relations, which for many years were characterised by a spirit of good neighbourliness and cooperation between partners, and had a mutually beneficial character".
Before launching the "special military operation" in Ukraine on 24 February, Putin had demanded that NATO end its enlargement into Eastern Europe and withdraw military infrastructure from countries that joined the bloc after 1997.
The Kremlin stressed Saturday that the conversation had been "frank" and had focused on Finland's plans to apply for NATO membership, which could be formalised at the June allied summit in Madrid, and the situation in Ukraine.
"In particular, Putin shared his views on the negotiating process between the Russian and Ukrainian delegations, which has been practically frozen by Kiev, which shows no interest in a serious and constructive dialogue," the official note said.
Finland's decision to join the Atlantic Alliance has come as a bitter blow to Russia, which has already threatened Helsinki with "military-technical" measures.
Russian Deputy Foreign Minister Alexandr Grushko warned on Saturday that NATO could deploy nuclear weapons in Finland and Sweden once both countries formally join NATO.
"It is enough to look at the map to understand how important allied enlargement is for Russia's security interests," he said.
He admitted that, for the moment, the Atlantic Alliance has not changed its nuclear policy, but its Secretary General Jens Stoltenberg has stated that "nuclear weapons can be placed closer to Russia's border and Polish leaders have assured that they are ready to receive them".
"If these statements are confirmed in practice, we will of course have to react with preventive measures to ensure a safe deterrent," the diplomat warned.
Despite Russia's ongoing military intervention in Ukraine, Grushko considered it "impossible" to suspect Russia of hostile intentions against Finland and Sweden, accusations he linked to attempts to "demonise" Russia politically and militarily.
Moscow, which accuses Helsinki of threatening Europe's security by opening a new allied flank in the north of the continent, cut off electricity supplies to the neighbouring country on Saturday, allegedly over non-payment problems.
Finnish membership would double the Russian Federation's border with the Atlantic Alliance, as Russia shares a 1,300-kilometre border with the Scandinavian country.
Russia now borders the following Western bloc members: Poland, Norway, Estonia, Latvia and Lithuania, as well as 49 kilometres of maritime border with the United States.