NATO foreign ministers and Montenegrin Prime Minister Milo Djukanovic on Thursday signed an accession agreement with Montenegro at the alliance headquarters in Brussels.
The signing is a first step towards making the Western Balkan country the 29th member of NATO. The country will become a full-fledged member once all 28 members ratify the protocol.
NATO's chief Jens Stoltenberg reiterated that the decision to invite Montenegro was not directed at Russia, that NATO is a defensive alliance meant to provide security.
Djukanovic said the move to accept his country into the alliance will bring stability to the Balkans.
Backed by an increase in US military spending, NATO is planning its biggest build-up in eastern Europe since the Cold War to deter Russia.
Moscow opposes any NATO extension to former communist areas of eastern and southeastern Europe, part of an east-west struggle for influence over former Soviet satellites that is at the center of the crisis in Ukraine.