Stockholm said on Wednesday that it was summoning the Russian ambassador to Sweden for warning the country would become a "legitimate target" of "retaliatory measures" if it joined NATO.
Sweden and neighbor Finland both ended decades of military non-alignment in May when they decided to join the Western defense alliance in the wake of Russia's invasion of Ukraine.
Russian ambassador Viktor Tatarintsev on Tuesday warned Finland and Sweden that by joining the alliance they both risked embroiling themselves in Moscow’s deepening confrontation with the West.
"If anyone still believes that this [NATO membership] in any way will somehow improve Europe's security, you can be sure that the new members of the hostile bloc will become a legitimate target for Russian retaliatory measures, including military ones," he wrote in a statement posted to the embassy’s website.
Tatarintsev said that rather than becoming safer, Sweden would be "taking a step towards the abyss."
"After the accession of Finland and Sweden, the total length of the border between Russia and NATO will almost double," he added.
In response, Sweden's Foreign Minister Tobias Billstrom announced that "the Ministry for Foreign Affairs will summon the Russian ambassador to make a clear statement against this blatant attempt at influence.”
"Sweden's security policy is determined by Sweden — no one else," Billstrom added.
NATO membership requires ratification by all 30 members of the military alliance. After having its bid ratified by Hungary this week, Finland is only waiting on Turkey, which has signaled it will soon give its approval.
Meanwhile, Sweden's bid has run into opposition from Hungary and Turkey after a litany of diplomatic spats.
Stockholm still hopes to join the bloc before the next NATO summit in Vilnius in July.