The expansion of NATO to Sweden and Finland poses "no direct threat for us... but the expansion of military infrastructure to these territories will certainly provoke our response," Putin said during a televised summit meeting of the Collective Security Treaty Organisation.

The Moscow-led military alliance includes six countries of the former Soviet Union: Russia, Belarus, Armenia, Kazakhstan, Kyrgyzstan and Tajikistan.

"This is a problem that is created completely artificially, because it is done in the foreign policy interests of the United States," Putin said, adding that NATO has become a "foreign policy instrument of one country".

"All this exacerbates an already difficult international security environment," Putin said.

Finland and Sweden are poised to jettison decades of military non-alignment to join NATO as a defence against feared aggression from Russia after Moscow sent troops into Ukraine on February 24.

Finland announced its intention to join NATO on Sunday as Sweden's ruling party said it backed membership, paving the way for a joint application.

One of Russia's justifications for the military campaign in Ukraine was the encroachment of NATO towards its western borders. However, Moscow will now see Finland, with which Russia shares a 1,300-kilometre (800-mile) border, join the alliance.

Speaking at the CSTO meeting hosted in Moscow, Belarusian President and close Putin ally Alexander Lukashenko was the only other leader from the six-nation bloc to address the NATO expansion and back military action in Ukraine.

"NATO is aggressively building up its muscles, yesterday drawing in the neutral Finland and Sweden," said Lukashenko, who in February allowed Russian troops to enter Ukraine from Belarusian territory.

He also accused Washington of a "desire to prolong as much as possible" the conflict in Ukraine.

"Without the speedy rallying of our countries... there may not be a tomorrow," Lukashenko said.

Earlier on Monday, the Kremlin said Finland and Sweden's NATO membership would not improve security in Europe, while Deputy Foreign Minister Sergei Ryabkov called it a "grave mistake with far-reaching consequences".