NATO's 28 members will meet Tuesday on Russia's incursion into Ukraine following a request by Poland.
"The developments in and arouund Ukraine are seen to constitute a threat to neighboring Allied countries and having direct and serious implications for the security and stability of the Euro-Atlantic area," NATO said in a statement released Monday.
The meeting comes after the defense alliance called Russia's staging of troops in Crimea "a breach of international law."
"It contravenes the principles of the NATO-Russia Council and the Partnership for Peace," the statement said.
NATO urged Russia and Ukraine to seek a peaceful resolution through dialogue, and through the dispatch of international observers under the auspices of the United Nations Security Council or the Organization for Security and Co-operation in Europe.