KIRKENES, Norway (Reuters) - Any expansion of NATO to include Sweden and Finland would upset the balance of power and force Russia to respond, Prime Minister Dmitry Medvedev said on Tuesday, underlining Moscow's nerves over moves to bring the Western alliance closer to its border.
Although Sweden and Finland are not actively seeking membership, both nations cooperate extensively with NATO and have openly debated the possibility of joining.
Speculation over Sweden mounted after it warned earlier this year that its defense capabilities were alarmingly limited. It was embarrassed in April when it could not respond to Russian military jets nearing its border, according to media reports.
Finland shares a long boundary with Russia.
"New participants emerging close to our border will change the parity and we'll have to take this into account and respond to that," Medvedev told a news conference at a summit of Nordic and Russian leaders in Norway.
"NATO ... has a military potential which can be used against our country in certain cases," Medvedev said, as Finnish Prime Minister Jyrki Katainen and Swedish Foreign Minister Carl Bildt looked on.
Russia has been concerned about NATO's expansion since former Soviet satellites began joining the Western alliance in the 1990s and has voiced particular opposition to the spread of military infrastructure closer to its borders.
Moscow has opposed the anti-missile shield the United States is deploying in Europe with help from NATO, saying it suspects its purpose is to weaken Russia's nuclear deterrent and upset the post-Cold War strategic balance.
"The fact that Norway is a member of NATO has not had any negative effect whatsoever on the development of cooperation in this part of the world," Sweden's Bildt said in response.
Danish planes flying from a NATO base in the Baltics eventually responded to the Russian planes approaching Sweden's border in April. The aircraft turned back just short of the boundary, according to a report by Swedish daily Svenska Dagbladet.
Sweden's alleged lack of military preparedness has been the subject of ridicule on Russian television, not least in the wake of remarks by Sweden's top military commander that the country could only fend off an invader for a matter of days before outside help was needed.
Sweden has forged particularly close ties with NATO since the end of the Cold War and regularly participates in security operation, including in Afghanistan.
(Reporting by Balazs Koranyi; additional reporting by Steve Gutterman in Moscos and Niklas Pollard in Stockholm; Editing by Alistair Scrutton)