Putin's statement was made during a meeting at the Kremlin's presidential palace with the Moscow-led security alliance CSTO, which consists of the former Soviet republics of Russia, Armenia, Belarus, Kazakhstan, Kyrgyzstan and Tajikistan.
"No threat to Russia"
Swedish and Finnish NATO membership does not pose a direct threat to Russia, according to the president.
"But an expansion of military infrastructure in this territory will certainly force a reaction from us"
It is not clear what type of reaction is meant.
Putin further said that the enlargement of NATO is being used by the United States to "aggressively" aggravate an already difficult global security situation.
Linde is looking for Lavrov
At three o'clock on Monday afternoon, the Swedish government announced that Sweden would apply for NATO membership.
Prime Minister Magdalena Andersson says that she does not intend to contact Vladimir Putin to announce Sweden's decision.
Foreign Minister Ann Linde states, however, that she has sought contact with Russian Foreign Minister Sergei Lavrov to have a conversation.
Ryabkov: "Serious mistake"
Russia's Deputy Foreign Minister Sergei Ryabkov commented on Sweden's and Finland's NATO plans before Putin's statement.
He considers membership to be a "serious mistake".
If NATO defines Russia as a "direct threat", the country will live up to it, says Ryabkov.
- The overall level of military tensions will increase, and there will be reduced predictability in this area.
It is a pity that common sense is sacrificed for an imaginary idea of what should be done under current conditions, says Sergei Ryabkov according to the Russian news agency Interfax.
- This is another serious mistake with far-reaching consequences.