Finland's flag will be hoisted outside Nato headquarters on Tuesday when it becomes the newest member of the Western alliance, Secretary General Jens Stoltenberg said on Monday.

"Tomorrow we will welcome Finland as the 31st member," Stoltenberg told reporters on the eve of a historic meeting of Nato foreign ministers in Brussels.

Russia's actions in Ukraine last year upended European security and prompted Finland – and its neighbour Sweden – to drop decades of non-alignment and seek to join Nato's umbrella.

Objections from Turkey and Hungary held up Helsinki's bid for months, and the two are still blocking Stockholm. The parliament in Ankara cleared the final obstacle for Finland with a vote last week.

Completing the ratification in well under a year still makes this the fastest membership process in the alliance's recent history.

Finland's President Sauli Niinisto will speak during formalities in Brussels, Helsinki said.

"President Putin [of Russia] went to war against Ukraine with a clear aim to get less Nato," Stoltenberg said. "He's getting the exact opposite." (AFP)