(L to R) Minister of Defense of Montenegro Milica Pejanovic and Foreign Minister of Montenegro Igor Luksic and NATO Secretary General Jens Stoltenberg take part in a Foreign Affairs meeting at the NATO headquarters in Brussels on December 2, 2015. REUTERS/John Thys/Pool NATO has officially invited the Balkan nation of Montenegro to join the alliance, the Associated Press reports.

The agreement will be the first step toward making Montenegro the 29th member of NATO, and it marks only the seventh time in history NATO has sought to expand.

"It is an historic day for the alliance, for Montenegro and for the stability of the Western Balkans," Stoltenberg told reporters in Brussels, the AFP notes.

"The enlargement of NATO with Montenegro confirms that NATO's door is open."

Montenegro's membership still hast to be ratified by all 28 other member states, as well as the US Senate, but until then it will be a party to NATO meetings, like July's coming Warsaw summit, as an observer.

Montenegro would be the third Balkan state in NATO, joining Croatia and Romania.

"I expect we will soon see 29 allied flags flying outside the NATO headquarters," Stoltenberg said.

NATO's expansion comes after Russia illegally annexed Crimea and used a form of hybrid warfare to destabilize Ukraine by backing anti-Western rebels.

Russia has repeatedly threatened military actions to counter the buildup of NATO, but top NATO officials say that Russia will have absolutely no say in Montenegro's decision.

"The fundamental principle is that every nation has the right to decide its own path ... including to decide what security arrangements it wants to be part of," Slotenberg said.

"Any sanctions or reactions from Russia will be absolutely unjustified, because it's about respecting the sovereign decision of a sovereign nation, Montenegro, to decide on its own path and that should be respected by everyone," Stoltenberg continued.

Additionally, Georgia, Macedonia, and Ukraine have expressed interest in joining NATO.