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

“This is a historic week,” Stoltenberg told reporters on the eve of a meeting of NATO foreign ministers in Brussels. “From tomorrow, Finland will be a full member of the alliance.” He said that he hopes Sweden will be able to join NATO in coming months.

The former Norwegian prime minister said that on Tuesday afternoon, “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.”

Stoltenberg said that Turkey, the last country to have ratified Finland’s membership, will hand its official texts to U.S. Secretary of State Antony Blinken on Tuesday. Stoltenberg said he would then invite Finland to do the same.

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

“It is a historic moment for us. For Finland, the most important objective at the meeting will be to emphasize NATO’s support to Ukraine as Russia continues its illegal aggression,” Haavisto said in a statement. “We seek to promote stability and security throughout the Euro-Atlantic region.”

Russia’s Deputy Foreign Minister Alexander Grushko said that Moscow would respond to Finland becoming NATO’s member by bolstering its defenses if needed.

“We will strengthen our military potential in the west and in the northwest,” Grushko said in remarks carried by the state RIA Novosti news agency. “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.”

The announcement of Finland’s entry comes just after Finnish voters gave a boost to conservative parties in a weekend election, depriving left-wing Prime Minister Sanna Marin of another term. Marin had championed her country’s NATO accession.

Fearing that they might be targeted after Russia invaded Ukraine a year ago, the Nordic neighbors Finland and Sweden abandoned their traditional positions of military non-alignment to seek protection under NATO’s security umbrella.

All 30 allies signed Finland’s and Sweden’s accession protocols. Turkey and Hungary delayed the process for months but have relented on Finland. Turkey has sought guarantees and assurances from the two, notably on tackling extremism. Hungary’s demands have never been explicit.

NATO must agree unanimously for new members to join. NATO officials are also keen to bring Sweden within the fold before U.S. President Joe Biden and his alliance counterparts meet in the Lithuanian capital Vilnius on July 11-12.

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