NATO Secretary General Jens Stoltenberg speaks during a media conference, ahead of a meeting of NATO foreign ministers, at NATO headquarters in Brussels, Monday, April 3, 2023 (AP)

Finland will become the 31st member of NATO on Tuesday, Secretary-General Jens Stoltenberg said Monday, prompting a warning from Russia that it would strengthen its defenses near their common border if the alliance deploys any troops in its new member.

Stoltenberg told reporters that this week is "historic" and starting tomorrow, "Finland will be a full member of the alliance." He hoped that in the coming months, Sweden will be able to join the alliance too.

"We will raise the Finnish flag for the first time here at the NATO headquarters. It will be a good day for Finland's security, for Nordic security, and for NATO as a whole", the former Norwegian prime minister said.

The office of Finnish President Sauli Niinisto said Finland would become a NATO member on Tuesday, April 4, 2023.

"President of the Republic of Finland Sauli Niinistoe will travel to Brussels and attend an accession ceremony at NATO Headquarters," the statement said.

Finland will submit its instruments of accession before the NATO Foreign Ministers meeting, according to the president's office.

The Turkish parliament on Thursday unanimously backed Finland's bid to join the US-led alliance only two weeks after Turkish President Recep Tayyip Erdogan openly blessed the bid.

Turkey will hand over its official texts to US Secretary of State Antony Blinken on Tuesday, Stoltenberg said, adding that he will later invite Finland to do the same.

Finnish President Sauli Niinisto, Defense Minister Antti Kaikkonen, and Foreign Minister Pekka Haavisto will attend the ceremony.

"We seek to promote stability and security throughout the Euro-Atlantic region," Haavisto said.

Russia's Deputy Foreign Minister Alexander Grushko said that Moscow would strengthen its defenses if needed, as a response to Finland becoming a NATO member.

"In case of deployment of forces of other NATO members on the territory of Finland, we will take addition steps to ensure Russia's military security," Grushko said in remarks carried by the state RIA Novosti news agency.

The announcement of Finland's accession comes right after Finnish voters gave a boost to conservative parties in a weekend election; the National Coalition of Petteri Orpo narrowly defeated the Social Democrats of outgoing Prime Minister Sanna Marin, who had supported her country's NATO accession.

"Sweden is not left alone. Sweden is as close as it can come as a full-fledged member," Stoltenberg said.

Finland and its neighbor Sweden ended decades of military non-alignment and decided to join the US-led alliance in the wake of the war in Ukraine.

Their applications were accepted at a June NATO summit. But the bids still needed to be ratified by all 30 of the alliance members' parliaments -- a process that got hung up once it reached the turn of Turkey and Hungary.

Accepting a new member into the 73-year-old alliance requires the unanimous approval of all 30 present members. NATO officials are also keen to bring Sweden within the fold before US President Joe Biden and his alliance counterparts meet in the Lithuanian capital Vilnius on July 11-12.