NATO Secretary General Jens Stoltenberg and Montenegro's Prime Minister Milo Djukanovic shake hands. | SAVO PRELEVIC/AFP/Getty Images Montenegro to join NATO Tiny Balkan state was seen as test case of NATO’s willingness to accept new members.

NATO foreign ministers gave the green light for Montenegro’s accession to the military alliance Wednesday, despite opposition from Russia, which considers the enlargement a “provocation.”

The integration of the 29th country into NATO is a “historic achievement,” NATO Secretary-General Jens Stoltenberg said in Brussels. “Successive rounds of enlargement have enhanced our security.”

Russian officials say the move is “irresponsible” and part of western efforts to "contain" Russia. Dmitri Peskov, spokesman for President Vladimir Putin, warned that the step would lead to a Russian response, but that it was too early to be specific.

"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 from the Russian side," he told reporters on Wednesday

The tiny state of 650,000 people, which was bombed by NATO in spring 1999, has become a test case of the alliance's willingness to accept new members, a step that Russia has fiercely resisted. Moscow has called the acceptance of former communist states into the alliance a threat to its security.

“NATO's door remains open [and] we support sovereign states to make their own choices,” Stoltenberg said, naming Bosnia-Herzegovina and Georgia as potential candidates for further enlargement. The inclusion of Georgia is certain to infuriate Russia, which fought a short war against Georgia in 2008 and has been adamant that the former Soviet Republic cannot join the alliance.

The issue is less fraught for Montenegro, which is a historic Russian ally but unlike Georgia does not share a border with Russia. However, Russia is the largest foreign investor in Montenegro, largely in real estate.

Despite those ties, Montenegro's parliament has already voted to join NATO. The government reformed the armed forces and its intelligence agencies to boost the chances of admission.

"This is a recognition of the very hard work for the alliance," Montenegro's Foreign Minister Igor Lukšić told reporters. He said there was still "much work ahead" in terms of reforms.

Several thousand people demonstrated in Montenegro in October, accusing the government of Prime Minister Milo Đukanović of corruption and mismanagement, while some also protested against joining NATO.

The military alliance had been split in recent years on whether to accept Montenegro. Some eastern members like Turkey, Croatia and Bulgaria had pushed the accession forward, while Germany and France were concerned it would upset Russia and endanger the fragile Minsk peace accord in Ukraine.

Moscow has close relations to many Slavic and Orthodox Christian Balkan countries, including Montenegro, and Stoltenberg said he wanted to avoid further tensions.

"The political contact to Russia remains open. We still use the NATO-Russia Council," Stoltenberg said.

“This is not designed as a message to Russia. It is not about Russia,” Douglas Lute, the U.S. ambassador to NATO, said ahead of the decision.

In an October visit to the country, Stoltenberg said the addition of the Balkan state would be a “win-win.”

“Countries which joined the alliance have been able to strengthen their democracy, boost their security and make their citizens safer,” he said.

Montenegro is also a candidate for EU membership.

“The NATO accession is generally a positive sign for the country’s future, since NATO membership has been in the past often a precursor for EU accession,” said Michael Benhamou, a former NATO political adviser, now working at the Martens Center, a Brussels-based think tank.

However, European Commission President Jean-Claude Juncker has said there will be no imminent enlargement.

Montenegro has already contributed about 45 soldiers to the NATO-led International Security Assistance Force in Afghanistan, and 15 soldiers to help train Afghan security forces in the subsequent NATO mission "Resolute Support.”

Its accession to the Atlantic alliance is expected to take at least one year.

"The accession talks can go quite quickly, we expect them to conclude early next year," Stoltenberg said. "But then all parliaments of 28 member states need to accept. So I cannot give a timeline on that."

This article has been updated to include context and comments by Dmitri Peskov