"We don't have problems with Sweden and Finland like we do with Ukraine," Putin told a news conference in the Turkmenistan capital of Ashgabat.

"We don't have territorial differences," the Russian leader continued.

"There is nothing that could bother us about Sweden and Finland joining NATO. If Finland and Sweden wish to, they can join. That's up to them. They can join whatever they want."

However, "if military contingents and military infrastructure were deployed there, we would be obliged to respond symmetrically and raise the same threats for those territories where threats have arisen for us," Putin said.

Sweden and Finland have both decided to apply to join NATO after Russia launched its military operation in pro-Western Ukraine on February 24.

The formal process for membership was launched at the NATO summit in Madrid on Wednesday.

Until now, Russia had always been critical of the prospect of the two Nordic countries joining the alliance, saying it would be a "destabilising factor" for international security.

Putin nevertheless condemned NATO's "imperial ambitions", accusing the alliance of seeking to assert its "supremacy" through the Ukraine conflict.

"Ukraine and the well-being of Ukrainian people is not the aim of the collective West and NATO but a means to defend their own interests," Putin said.

"The NATO countries' leaders wish to... assert their supremacy, their imperial ambitions," he added.

The Atlantic alliance and "above all the United States have long needed an external enemy around which they can unite their allies," the Russian leader said.

"Iran wasn't good for that. We've given them this opportunity... to gather the whole world around them."