Nato foreign ministers and Montenegrin Prime Minister Milo Djukanovic signed Thursday 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 and 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 U.S. military spending, NATO is planning its biggest build-up in eastern Europe since the Cold War to confront what Washington calls “Russian aggression”.

Moscow opposes any NATO extension to former Soviet states in eastern and southeastern regions of Europe, and says the U.S.-led alliance is seeking to undermine Russian interests in the region and starting a new Cold War.