Finland will almost certainly apply for membership in NATO, Sweden's Foreign Minister Ann Linde told Swedish television today.

Russia's invasion of Ukraine has forced both Sweden and its eastern neighbour Finland to review beliefs that military neutrality is the best means of ensuring national security.

"We know more or less that they (Finland) will apply for NATO membership. And that changes the whole balance... If one of our countries join, we know that tensions would increase," Mr Lindetold public broadcaster SVT.

Asked whether she thinks Finland will join NATO, Linde said: "I think you can say that quite surely."

Both Sweden and Finland are expected to make a decision on whether to join the military alliance in the coming weeks.

Finland's Foreign Minister Pekka Haavisto said on Friday that he hoped Finland and Sweden would make similar decisions at the same time.

Earlier today it was revealed that Denmark and Sweden are summoning Russia's ambassadors after a Russian spy plane violated the air space of both countries.

Officials said the plane entered Danish airspace on Friday evening east of the Danish Baltic island of Bornholm before flying into Swedish airspace.

"The Russian ambassador is summoned to the foreign ministry tomorrow," Danish Foreign Minister Jeppe Kofod tweeted today, referring to a "new Russian violation of Danish airspace".

The Swedish foreign ministry also said the Russian ambassador would be summoned in Stockholm.

"There exist established procedures for this kind of case. It concerns notably summoning the representative of the implicated nation to the foreign ministry," it said in an email.

Mr Kofod added it was "totally unacceptable and particularly worrying in the current situation", alluding to Russia's invasion of Ukraine and rising tensions with NATO, where Denmark is a member.

Danish Foreign Minister Jeppe Kofod (file image)

Henrik Mortensen, a Danish Defence Command press officer, told AFP about the incident.

"It was a reconnaissance plane that was in our airspace for a very brief moment. Two Danish F-16 immediately intervened," Mr Mortensen said, adding that such incidents are rare.

Denmark is a member of NATO, unlike Sweden where a debate is taking place over whether it should abandon its non-aligned status and join the alliance.

Questioned by the Dagens Nyheter newspaper, the Swedish defence minister said there was no proof that the breach was linked to current discussions on Stockholm eventually joining NATO.

Russia has already signaled that Stockholm and Helsinki, which is also contemplating membership, should consider the consequences of such a move on bilateral relations and Europe's overall security architecture.