NATO has formally invited Montenegro to become its 29th member, marking the seventh expansion in its history.

NATO foreign ministers have signed an accession protocol with the small Balkan state. The decision needs to be formally ratified in each of the alliance's national parliaments, and by the US Senate. NATO Secretary-General Jens Stoltenberg said it was the "beginning of a new secure chapter" in the former Yugoslav republic's history.

Over Russia's objections, Montenegro's prime minister Milo Dukanovic says his country will stand "shoulder to shoulder" with other member of the US-led alliance. Montenegro has a tiny population and an active-duty military of about 2,000 memembers. The last new recruits to the 28-member alliance were Albania and Croatia in 2009.