NATO will hold its second emergency meeting in 48 hours on the Ukraine crisis on Tuesday after Poland requested "article 4" consultations with its allies on the threat to its security.
The 28 member states' ambassadors will meet because "developments in and around Ukraine are seen to constitute a threat to neighbouring allied countries and having direct and serious implications for the security and stability of the Euro-Atlantic area," NATO head Anders Fogh Rasmussen said on Monday.
"Under article 4 of the Treaty, any ally can request consultations whenever, in the opinion of any of them, their territorial integrity, political independence or security is threatened," the statement said.
NATO added that Rasmussen has also contacted the Russian ambassador to the alliance to suggest "an extraordinary meeting of the NATO-Russia Council on the situation in Ukraine on Wednesday."
The ambassador "promised to consider this proposal and come back" to Rasmussen on Tuesday, another statement said.
Tuesday's meeting follows a similar gathering at NATO headquarters in Brussels Sunday. Rasmussen said he had called that meeting on his own initiative and not under Article 4.
Article 4 meetings are quite rare but have been held most recently on Turkey's fears of spillover from the bloody conflict in Syria.
Article 5 of NATO's Washington Treaty can be invoked if one member has been the victim of an armed attack, which is considered to be an attack on all and requires a military response by all.
The only time Article 5 has been invoked was after the 9/11 terror attacks on the United States.
Rasmussen on Sunday described the situation in Ukraine as a threat to Europe's peace and security, and called on Russia to reverse course following its military incursion into Crimea.
Ukraine signed up to a partnership deal with NATO in 1997 but is not a full member.
It was given the nod to eventually join at a 2008 summit in a move that infuriated Russia but two years later, now ousted pro-Russian leader Viktor Yanukovych ditched this option.