NATO has invited the former Yugoslav state of Montenegro to join the alliance.

"NATO foreign ministers have just taken the historic decision to start accession talks for Montenegro to become the 29th member," Secretary-General Jens Stoltenberg said.

On Monday, a US State Department spokeswoman said Washington backed Montenegro's invitation to join.

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

The invitation by the US-led military alliance has angered Russia which has repeatedly said the move is a threat to stability in the Balkan country that is a popular holiday destination and investment area for richer Russians.

On hearing the news, the Kremlin said NATO's continuing expansion to the East would lead to retaliatory measures.

The Russian foreign ministry last week said such an invitation would be "yet another serious blow" to the existing security system, adding it would be "confrontational" and "could lead to further complication of the already difficult relations between Russia and NATO."

Russian MP Sergei Zheleznyak said on Monday that Russia would have to reconsider its relations with Montenegro if the country 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."

US ambassador to NATO Douglas Lute said on Monday that he "understood Russia's perspective on this," but insisted the accession of Montenegro was "not designed in any way as a message to Russia."

He also accepted that public opinion in Montenegro was not necessarily fully supportive of the country's entry into the alliance.

Fellow Balkan countries Albania and Croatia were admitted in 2009 - Macedonia, Bosnia-Herzegovina and Georgia are also being considered for inclusion.

Montenegro's accession was one of a number of topics under discussion at the two-day meeting of the organisation's top officials in Brussels.

Syria, Turkey and Afghanistan were also on the agenda.

"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," Mr Stoltenberg said.

(Sky News)