Finland and Sweden joining the Nato alliance is "not a problem" for Russia, according to Russian President Vladimir Putin, as he said the decisions by both Nordic nations are related to internal issues.

"With Sweden and Finland, we don't have the problems that we have with Ukraine. They want to join Nato, go ahead," Putin told Russian state television.

However, news agency Reuters reported that Putin also warned Russia would respond if Nato deployed troops and infrastructure in Finland and Sweden.

"But they must understand there was no threat before, while now, if military contingents and infrastructure are deployed there, we will have to respond in kind and create the same threats for the territories from which threats towards us are created," Putin said, adding that it was "inevitable" that relations between Russia, Finland and Sweden would now become more tense.

Former Russian President Dmitry Medvedev has previously warned that Russia would have to bolster its defences in the Baltic Sea region, including by deploying nuclear weapons, if Finland and Sweden were to join Nato.

Tuomas Forsberg, a Professor of International Politics at the University of Tampere, told Yle that Putin's rhetoric is beginning to focus more on the possibility that Nato's military infrastructure would be brought to Finland and Sweden, rather than membership as such.

"Putin is now the good cop and Medvedev has become the bad cop, whereas perhaps in the past it was thought to be the other way round," Forsberg said.

Nato countries decided to officially invite Finland and Sweden to join the alliance on Wednesday, with Finland's President Sauli Niinistö telling reporters that the accession protocol will be signed by next Tuesday at the latest.

All 30 current Nato member states must formally ratify Finland and Sweden's applications, which Forsberg believes will take at least a few months, even on an accelerated timetable.

"Some countries will ratify very quickly, but there is always that one last country. Often it can take up to a year," he noted.

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