BRUSSELS, Belgium - Foreign ministers from NATO countries were expected to invite Montenegro to join the military alliance despite Russia's objection to the move, diplomats said, the latest sign of discord between the West and Moscow even as they both battle the Islamic State in Syria and Iraq.

Montenegro's accession was one of several topics being discussed at a two-day meeting of top diplomats from the alliance that began Tuesday. Other issues include the funding for NATO's mission in Afghanistan, improving Turkish defenses, and injecting "predictability" in relations with an increasingly assertive Russia in places such as Ukraine and Syria, NATO Secretary-General Jens Stoltenberg said.

"The security environment in which we meet today is dark: Terrorist attacks, violent instability [and] the breach of international rules. These are serious challenges from many different directions, and NATO is responding," Stoltenberg said. He pointed to violent extremism in the "South" - a NATO buzzword for the Middle East and North Africa - and said recent extremist attacks show a "qualitatively new challenge."

The decision on a formal invitation to Montenegro - part of the alliance's long-standing "open-door" policy for potential new members - comes Wednesday, Stoltenberg said, but two NATO diplomats said it's a done deal. Russia has warned Montenegro that its joining NATO would be regarded as a provocation.

On Monday, U.S. State Department spokeswoman Elizabeth Trudeau said Washington supports an invitation to Montenegro.

"We believe Montenegro's membership in NATO will contribute to Balkan and European security," she said.

Accession by Montenegro is on track to take place at the alliance's summit in Warsaw next year, about seven years after NATO last expanded by taking in Albania and Croatia.

Last week, Russian Foreign Ministry spokeswoman Maria Zakharova said at a briefing that a decision by NATO to start negotiations with Montenegro on its accession to the alliance would be "yet another serious blow" to the existing security system. She said such a move "could lead to further complication of the already difficult relations between Russia and NATO."

Sergei Zheleznyak, a prominent member of Russia's parliament, on Monday said Russia would have to reassess its relations with Montenegro if it joined NATO without holding a referendum.

"We would have to change our policy in regard to this friendly country," he said. "If NATO military infrastructure were placed there, we would have to respond by limiting our contacts in economic and other spheres."