Nato chief Anders Fogh Rasmussen urged Russia on Sunday to stop its military activity and threats against Ukraine, saying Moscow's action threatened "peace and security in Europe."
"Russia must stop its military activity and its threats," he said in a brief statement before opening crisis talks with Nato's 28 ambassadors. "Today we will discuss the implications for European security."
Speaking as Ukraine's interim authorities accused Russia of triggering "a red alert" with what was "actually a declaration of war", Rasmussen said he had convened the North Atlantic Council "because of President Putin's threats against this sovereign nation."
"What Russia is doing now in Ukraine violates the principles of the United Nations Charter. It threatens peace and security in Europe. Russia must stop its military activities and its threats."
"We support Ukraine's territorial integrity and sovereignty. We support the right of the people of Ukraine to determine their own future without outside interference.
"And we emphasise the need for Ukraine to continue to uphold the democratic rights of all people and ensure that minority rights are protected.
"Ukraine is our neighbour, and Ukraine is a valued partner for Nato."
It was urgent to de-escalate "this dangerous situation," he concluded.