BRUSSELS—The North Atlantic Treaty Organization is poised this week to move forward on offering membership to Montenegro, the alliance's first expansion since 2009.

The application by Montenegro, a small Balkan country, has become a source of friction between Moscow and the Western alliance. Russia has repeatedly criticized previous rounds of NATO expansion, and has said allowing in Montenegro would be a strategic threat.

Foreign ministers for the alliance, gathering in Brussels this week, will decide on whether to formally move forward with offering membership to Montenegro at the summit of NATO leaders in July in Warsaw. Alliance officials said Monday that while a formal vote of foreign ministers is needed, an invitation to Montenegro is all but ensured.

NATO Secretary-General Jens Stoltenberg said the vote by foreign ministers, which must be unanimous, would show that the alliance is open to continued expansion. "Extending an invitation to start accession would be a historic decision, it will signal our continue commitment to the western Balkans and our open-door policy," he said.

Albania and Croatia, two other Balkan countries, were the last countries to join the alliance in April 2009.

Other would-be applicants, most notably Georgia, aren't going to see their applications advance. Many NATO members say that because Russia occupies portions of Georgia, the country cannot be offered membership now, despite its participation in many alliance operations.

Russia, according to NATO officials, has sent letters to several allies warning that expanding to include Montenegro would be destabilizing for the region and would be unhelpful.

Vesko Garcevic, Montenegro's national coordinator for NATO membership, said they were aware of the letters from Russia. But he said there was broad and growing support for membership from Montenegrins, including from a majority in Parliament.

Alliance members said the Balkan countries have committed long ago toward integrating with the European Union. One allied official said that allowing Montenegro wouldn't pose any new challenges to Russia.

"This is not a major upheaval of the international strategic order," an official said.

Douglas Lute, the U.S. NATO ambassador, said in a press briefing Monday that Russian officials wouldn't be in the room Wednesday voting on Montenegrin membership.

"I understand Russia's perspective on this, but this is not about Russia," Mr. Lute said. "This is about Montenegro, which, by the way, is hundreds of miles away."

Some alliance officials say Russia has fomented opposition to NATO within Montenegro, supporting and encouraging protests, for example.

A Russian official denied Moscow had interfered in Montenegrin affairs but said Russia had long opposed NATO's open-door policy, arguing that it didn't make Europe more secure.

Mr. Stoltenberg didn't address the accusations of Russia encouraging opposition but said protests didn't worry him.

"There are different opinions … that is part of being a democratic society," he said. "To have some people demonstrating and showing their opinion is something normal in democratic society."

Montenegrin officials said that Russian opposition should be of no surprise, but shouldn't deter members.

"Russian has been opposing NATO enlargement for years," Mr. Garcevic said. "We would like Montenegro be part of NATO and the EU. This is where Montenegro should belong."

Write to Julian E. Barnes at julian.barnes@wsj.com

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(END) Dow Jones Newswires

November 30, 2015 21:45 ET (02:45 GMT)