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NEW: A top U.S. general says "timing is right" for Montenegro; Russia shouldn't have "a veto" over NATO invitations

Kremlin spokesman says the invite will spur a "response"; another official says military, technical cooperation will end

CNN  — 

NATO has formally invited Montenegro to join the alliance, a move that’s spurred threats from Russian officials at loggerheads with NATO over everything from Ukraine to Syria to Turkey’s downing of a Russian warplane.

The official invitation, announced Wednesday by NATO Secretary-General Jens Stoltenberg, triggers the start of accession talks, according to the alliance.

While it comes at a time of heightened tensions between NATO and Russia, it didn’t happen out of the blue; it’s the result of a process that began nine years ago.

“Today, we proudly receive a #NATO membership invitation,” said Montenegro’s President Milo Djukanovic, according to his government’s Twitter feed. “This is a historic day for #Montenegro. The most important (since) the 2006 (independence) referendum.”

As expected, this celebratory sentiment wasn’t echoed in Russia.

“Moscow has always noted at various levels that the continuing expansion of NATO and NATO’s military infrastructure to the East, of course, cannot but lead to response actions from the East, namely, the Russian side in ensuring security interests and supporting the parity of interests,” Kremlin spokesman Dmitry Peskov said, according to the state-run Tass news agency.

Peskov declined to say what Russian President Vladimir Putin will do about the Montenegro invitation, saying, “Now, there are other priorities.”

But Viktor Ozerov, the chair of the Federation Council’s Committee on Defense and Security, told another state news agency that Russia will end military and technical cooperation with Montenegro if its membership becomes official.

“Montenegro should recognize that a lot of programs that have been previously realized by it with Russia … will be impossible in the context of its NATO accession,” Ozerov told RIA Novosti.

Kerry: ‘NATO is not a threat to anybody’

Twenty-eight countries, from the United States and Canada on one side of the Atlantic to a host of nations on the other, currently make up NATO. Twelve have been part of the bloc since its inception in 1949, in the thick of the Cold War, as a bulwark against the then-Soviet Union, and its membership has expanded periodically since.

Its biggest single push came in 2004 with the addition of Bulgaria, Estonia, Latvia, Lithuania, Romania, Slovakia and Slovenia. Albania and Croatia joined NATO five years later. Like Montenegro, all those countries are in Eastern Europe and have historic ties to Moscow.

U.S. Secretary of State John Kerry took pains Wednesday to point out that Montenegro’s possible accession to NATO “is not new,” having been in the works for years.

Still, Wednesday’s announcement comes at a particularly strained time in NATO-Russia relations.

The two sides have been at odds over Ukraine, where protesters pushed out the president about two years ago because of his closeness to Russia. The country has been boiling ever since, with Kiev and its Western allies – including NATO, even though Ukraine isn’t a member – blasting Moscow for annexing Crimea and fomenting unrest elsewhere, while Russia says it favors popular determination for Ukraine and downplays its military’s role in armed conflict.

Another major area of contention is Syria. Putin has been one of Syrian President Bashar al-Assad’s strongest (and few) allies in that country’s bloody, years-long civil war, at a time when many, including U.S. President Barack Obama, have called for Assad’s ouster and supported moderate rebels fighting him.

There have been many spillover effects from this conflict, the latest – and most perilous to Russia-NATO relations – coming when Turkey, a NATO member, shot down a Russian warplane. (Exactly where is a point of contention, with Moscow claiming the aircraft was struck in Syrian airspace and Ankara saying it was over Turkey. The nations also give conflicting reports on whether any warnings went out beforehand.)