The US Defence Department announced late Monday that it is suspending exercises and other activities with the Russian military over Moscow's military involvement in Ukraine and asked the Vladimir Putin government to take steps to de-escalate tension in the region.
Pentagon spokesman, Navy Rear Adm. John Kirby, said that the US military has "put on hold'' all military-to-military engagements, including bilateral meetings, port visits and planning conferences.
Kirby said the Pentagon values the relationship it has developed with the Russian military in the last few years. Kirby further stated that the US urges the Russian forces in the Crimea region of Ukraine to return to their bases immediately.
Ukraine on Monday accused Russia of pouring more troops into Crimea and giving its forces an ultimatum to surrender as world leaders grappled with Europe's worst standoff since the Cold War.
Ukraine's UN ambassador, Yuriy Sergeyev, accused Russia of "an act of aggression", telling the Security Council that Russia has deployed some 16,000 troops from Russian territory to Crimea since February 24.
However, Russia's Black Sea Fleet swiftly denied any such demand as "nonsense" and the country's Parliament Speaker said there was no need yet for Moscow to use its "right" to launch military action in Ukraine.
In another development, Moscow's UN ambassador claimed that ousted Ukrainian president Viktor Yanukovych had requested Russian President Vladimir Putin in a letter to use force to help restore law and order in his turmoil-stricken country.
Vitaly Churkin showed a copy of the letter to the members of the UN Security Council as he spoke at an open council meeting on the current crisis in Ukraine.
"People are persecuted on political and language grounds," the Russian permanent representative to the UN quoted the letter, which was dated March 1, as saying.
"In this context, I appeal to the President of Russia, Vladimir Putin, to use the armed forces of the Russian Federation to re-establish the rule of law, peace, order, stability and to protect the people of Ukraine," he continued.
Moscow, said the envoy, still recognizes Yanukovych, who was removed from power by the Ukrainian Parliament last month, as Ukraine's legitimate leader, rather than interim President Oleksandr Turchynov.
Churkin also criticized some Western UN ambassadors for getting information about Ukraine only "from US TV".
Reading the letter to reporters after the council meeting, Churkin said Ukraine has plunged into "chaos and anarchy" and "is in the grip of outright terror and violence driven by the West".
Today's emergency meeting of the North Atlantic Treaty Council (NATO), which includes the ambassadors of all 28 NATO Allies, has been convened at the request by Poland under article 4 of NATO's founding Washington Treaty, reports claimed.
Under article 4 of the Treaty, any member country of the bloc can request consultations "whenever, in the opinion of any of them, their territorial integrity, political independence or security is threatened".
The NATO said the 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".
Crimea, a multi-ethnic region with a large Russian population, enjoys a high degree of autonomy after Ukraine gained independence from the Soviet Union in August 1991. Russia has maintained its only Black Sea naval base in the port of Sevastopol, Crimea.