LONDON, May 16 (Reuters) - President Vladimir Putin said on Monday that there was no threat to Russia if Sweden and Finland joined NATO but cautioned that Moscow would respond if the U.S.-led alliance bolstered military infrastructure in the new Nordic members.

Putin, Russia's paramount leader since 1999, has repeatedly cited the post-Soviet enlargement of the NATO alliance eastwards toward Russia's borders as a reason for the conflict of Ukraine.

But Putin, who has in recent months rattled Russia's nuclear sabre at the West over Ukraine, made an unusually calm response to Finland and Sweden's bids to join NATO, the biggest strategic consequence of Russia's invasion of Ukraine to date. read more

"As to enlargement, Russia has no problem with these states - none. And so in this sense there is no immediate threat to Russia from an expansion (of NATO) to include these countries," Putin told the leaders of a Russian-dominated military alliance of former Soviet states.

Putin, though, laced his newly found tranquillity on NATO with a warning.

"But the expansion of military infrastructure into this territory would certainly provoke our response," Putin said.

"What that (response) will be - we will see what threats are created for us," Putin told the leaders of the Collective Security Treaty Organization (CSTO), which includes Belarus, Armenia, Kazakhstan, Kyrgyzstan and Tajikistan.

The Kremlin chief's remarkably serene response to one of Russia's most sensitive geopolitical worries - the post-Soviet enlargement of NATO - contrasted to some tougher language from his foreign ministry and senior allies.

Before Putin spoke, Russian Deputy Foreign Minister Sergei Ryabkov said the West should have no illusions that Moscow would simply put up with the Nordic expansion of NATO. Those comments were still being played up on state television.

One of Putin's closest allies, former President Dmitry Medvedev, said last month that Russia could deploy nuclear weapons and hypersonic missiles in the Russian exclave of Kaliningrad if Finland and Sweden joined NATO.