The Nordic neighbours ended decades of military non-alignment last May when they decided to join the Atlantic alliance in the wake of Russia's invasion of Ukraine.

Finland and Sweden will become "legitimate targets" of Russian "retaliatory measures" once they join NATO, Russian ambassador Viktor Tatarintsev warned Tuesday.

But Sweden's Foreign Minister Tobias Billstrom said "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.

The Russian diplomat upped the ante in the standoff with Stockholm in a statement posted on his embassy's website.

"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 warned.

He said that rather than becoming safer Sweden was "taking a step towards the abyss" by joining NATO.

"After the accession of Finland and Sweden, the total length of the border between Russia and NATO will almost double," he added.

NATO membership requires ratification by all 30 members of the bloc. After having its bid ratified by Hungary this week, Finland is only waiting on Turkey, which has signalled it will approve it shortly.

Sweden's bid has meanwhile run into opposition from Hungary and especially Turkey after a litany of diplomatic spats.

Stockholm still hopes to join before the next NATO summit in Vilnius in July.