NATO began talks Monday with Montenegro over possibly becoming the 29th member of the US-led alliance currently embroiled in a crisis with Russia over Ukraine.

NATO formally invited the small Balkan state to join in December, sparking a sharp response from Moscow which warned it would have to respond to what it sees as encroachment on a region crucial to its security.

NATO officials said two days of talks in Brussels will cover the "details of membership including political, military and legal questions, and provide an opportunity for both sides to clarify outstanding issues."

The meeting was a "mark of the progress made by Montenegro since regaining its independence. NATO membership will reinforce Montenegro's security and sovereignty," NATO negotiator Thrasyvoulos Terry Stamatopoulos said in a statement.

During the accession process, which NATO chief Jens Stoltenberg hopes to conclude this year, Montenegro will have to push through a series of reforms to ensure it meets the democratic and civil society norms expected in the alliance.

Montenegro Prime Minister Milo Djukanovic has strongly backed NATO membership but the pro-Russian opposition has called for his ouster.

Djukanovic won a vote of confidence in Parliament last month, insisting the government remained "unconditionally attached" to its goal of NATO membership.

Stoltenberg says Montenegro is free to choose NATO membership and that the December announcement was "not directed at anyone."

Among the other states of the former Yugoslavia, Croatia and Slovenia have both joined NATO, while many of the former communist states in eastern Europe once ruled from Moscow have also become members.

Russia sees this process, backed by a military build-up in response to the Ukraine crisis, as firmly directed against its security.