Finland will become a member of NATO on Tuesday, completing a historic security policy, while neighbour Sweden is kept in the waiting room, News.az reports citing Reuters.

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.

"It will be a good day for Finland's security, for Nordic security and for NATO as a whole," NATO Secretary-General Jens Stoltenberg told reporters on Monday.

The event marks the end of an era of military non-alignment for Finland.

But war in Ukraine, which began in February 2022, prompted Finns to seek security under the umbrella of NATO's collective defence pact, which states that an attack on one member is an attack on all.

Sweden underwent a similar transformation in defence thinking and Stockholm and Helsinki applied together last year to join NATO.

But Sweden's application has been held up by NATO members Turkey and Hungary.

After both those countries approved Finland's application last week, the final formal step on Helsinki's journey will come when Foreign Minister Pekka Haavisto hands his nation's accession document to US government officials in Brussels.

Finland's flag will then be raised outside NATO headquarters alongside those of the alliance's 30 other member countries before a gathering of NATO foreign ministers.