NATO member states Poland, Estonia, Latvia and Lithuania have triggered NATO Article 4 to launch consultations within the alliance over Russia’s attack on Ukraine, writes CNN.

“The Parties will consult together whenever, in the opinion of any of them, the territorial integrity, political independence or security of any of the Parties is threatened”, - Article 4 of The North Atlantic Treaty says.

According to the NATO website, consultation under Article 4 can lead to collective action among the 30 member states.

The website says Article 4 has been invoked six times previously since the alliance formed in 1949, most recently by Turkey in February 2020 after dozens of Turkish soldiers were killed by an attack by Syrian government forces in opposition-held areas of northern Syria.

Article 4 is separate from Article 5, which is the alliance’s declaration that an attack against one member is considered an attack against all.