Kremlin spokesperson Dmitry Peskov said on Thursday that if Finland and Sweden joined NATO then Russia would have to "rebalance the situation" with its own measures.
Moscow's invasion of Ukraine, which it says aims among other things to degrade Ukraine's military potential and prevent it becoming a bridgehead for a NATO attack, has prompted the two Nordic countries to consider joining the U.S.-led alliance.
If the two countries join, "we'll have to make our western flank more sophisticated in terms of ensuring our security," Peskov told Britain's Sky News.
However, he said Russia would not see such a move as an existential threat, of the kind that might prompt it to consider using nuclear weapons.
Russia's invasion of Ukraine on Feb. 24 and the resulting fighting has caused the deaths of thousands of civilians and soldiers and the devastation of towns and settlements, as well as triggering an unprecedented barrage of coordinated Western sanctions.
(Writing by Kevin Liffey; Editing by Mark Trevelyan)
