The wary Nordic country made the construction of emergency shelters mandatory under apartment blocks and office buildings as early as in the 1950s, which explains their current high number.

It will be Green Party member Haavisto's third attempt at becoming Finland's head of state, having been the runner-up in both 2012 and 2018.

Russia's invasion of Ukraine convinced Sweden and Finland last year to ditch long-held policies of military non-alignment and seek security of Nato's collective defence commitment.

It says it is 'within reach' for Sweden to join in time for the July 11-12 Nato summit in the Lithuanian capital Vilnius.

Finland's accession roughly doubles the length of the border that Nato shares with Russia and bolsters its eastern flank as the war in Ukraine grinds on with no resolution in sight.

The military alliance will welcome Finland as its 31st member in a flag-raising ceremony at Nato headquarters on the outskirts of Brussels, attended by Finnish President Sauli Niinisto and government ministers.

The Turkish parliament was the last among the 30 members of the alliance to ratify Finland's membership after Hungary's legislature approved a similar bill.

New Nato entrants must be approved by all existing members of the Western military alliance, and support for Finland's application remains pending from Turkey and Hungary.

Spooked by Moscow's invasion of Ukraine, Finland and Sweden dropped their decades-long policies of military non-alignment and applied to join the North Atlantic Treaty Organization in May last year.

The Scandianavian nations' bids to join the 30-member alliance, which is arming Kyiv against President Vladimir Putin's army, have been ratified by all but Hungary and Turkey.

Erdogan's comments during a televised meeting with younger voters came days after Ankara suspended Nato accession talks with the two countries.

Finland has a history of wars with Russia, but the forest-covered border is still only marked with signs and plastic lines for most of its 1,300km length.