NATO member states formally invited Montenegro to join the alliance, drawing a prompt response from Russia that it plans to suspend cooperation with the tiny Adriatic nation. US Secretary of State John Kerry insisted that NATO is "not a threat to anybody."

Alliance Secretary-General Jens Stoltenberg said the invitation to Montenegro reaffirmed NATO's "open door" policy toward potential member states including Georgia, and promised a statement later with "renewed commitments to our strong support of Georgia's aspiration for NATO membership" '€” another move that could rankle Moscow.

Stoltenberg announced the "historic" invitation to Montenegro on Wednesday, the second day of a two-day meeting of NATO foreign ministers.

"Today, we took another step toward the full integration of Europe and toward the common defense by inviting Montenegro into the alliance," Kerry said.

The planned expansion of the alliance rooted in the Cold War, which grew to 28 members in 2009, comes amid rising tensions between NATO and Russia over a variety of issues including Syria, the conflict in eastern Ukraine, Russia's annexation of Crimea and a new Russian assertiveness in the skies in northern Europe.

Russia has opposed the NATO accession of Montenegro, a favored getaway spot and investment site for some Russians. President Vladimir Putin's spokesman, Dmitry Peskov, told reporters that Russia will be looking at possible retaliatory measures.

Viktor Ozerov, chairman of the defense committee at the upper house of the Russian parliament, told state-owned RIA Novosti news agency on Wednesday that Russia will freeze joint projects with Montenegro including defense cooperation.

Kerry sought to dispel Moscow's concerns.

"NATO is not a threat to anybody," he said. "It's not an offensive organization. It's a defensive alliance. It is meant to simply provide security. It's not focused on Russia per se or anybody else."

Montenegrin Defense Minister Milica Pejanovic Djurisic said the invitation "is a confirmation of our great progress in the sectors of defense and other areas relevant to the Alliance accession."

In 1999, Montenegro, then in a union with Serbia, was heavily bombarded in the first waves of NATO airstrikes which were triggered by Serbia's violent crackdown against independence-seeking Kosovo Albanians. During the three-month bombardment, Montenegro became refuge for Serbia's pro-Western opposition leaders and dissidents who were persecuted by Serbia's then-leader, Slobodan Milosevic

Srdjan Milic, leader of Montenegro's pro-Russian opposition, said NATO's formal invitation "represents an aggression on peace, stability and security of citizens of our country."

Several recent protests by thousands in Montenegro against the pro-NATO government have turned violent.

The announcement sets in motion an accession process that will continue over months before Montenegro formally joins. Until all NATO states ratify the decision, Stoltenberg said Montenegro will be a non-voting participant in meetings.

The last members to join NATO were fellow Balkan countries Albania and Croatia in 2009.

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Dusan Stojanovic in Belgrade, Nataliya Vasilyeva in Moscow and Bradley Klapper in Brussels contributed to this report.

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