How do you join NATO and how close is Ukraine to becoming a member?

Russian President Vladimir Putin has demanded that NATO not expand to Ukraine or any former Soviet states over, expressing concern over the extension of a military and political alliance founded to counter the former Soviet Union.

Some claim this issue is at the heart of why 150,000 Russian troops are massed near the Ukrainian border, sparking warnings of an imminent invasion. Moscow denies this.

But how does a country join NATO and how far away is Ukraine from becoming a member?

The North Atlantic Treaty was signed in 1949 by 12 founding countries as an alliance to counter the Soviet Union.

The short treaty commits members to protect each other if another is threatened.

"An armed attack against one...shall be considered an attack against them all," the treaty's Article 5 states.

That article was first triggered after the terrorist attacks on September 11, 2001, against the United States with the invasion in Afghanistan.

NATO has expanded to include 30 countries over the past 70 years and members may invite other European countries to join the organisation.

Those potential members must follow democratic values and contribute to Euro-Atlantic security in order to be considered.

Invitees will receive a membership action plan that advises them on how to become a member, a step that's followed by accession talks.

An accession protocol to the treaty must then be signed unanimously by NATO members in order for the country to become part of the organisation.

At the 2008 NATO summit in Bucharest, the alliance welcomed Ukraine and Georgia’s aspirations for membership in the alliance and agreed that "these countries will become members of NATO".

But the countries were not offered membership action plans, with NATO countries stating they would