The United States has condemned Russia's military intervention into Ukrainian territory, and called on Russia to withdraw its forces back to bases in Crimea.
The White House said on March 1 that President Barack Obama had a 90-minute phone call with Russian President Vladimir Putin in which he said the appropriate way to address any concerns "is peacefully through direct engagement" with the Ukrainian government and international mediating bodies.
It also warned that continued violation of international law by Moscow would lead to further "political and economic isolation."
The Kremlin said Putin told Obama that Moscow will reserve its right to "protect its interests and Russian-speaking populations" in Ukraine.
Ukrainian acting President Oleksandr Turchynov has ordered Ukraine's armed forces to be put on combat alert.
The announcement came on March 1 after Russia's upper house of parliament voted unanimously to approve the use of Russian forces in Ukraine.
Russian President Vladimir Putin had asked the Federation Council for authorization to deploy troops in order to protect ethnic Russians in Ukraine and Russian naval forces based in Ukraine's Crimea region.
In a televised statement, Turchynov said authorities were also increasing security around Ukrainian nuclear facilities, airports, and other "strategic" facilities. Turchynov said there was no justification for what he called Russian aggression.
Ukraine's Prime Minister Arseny Yatsenyuk said any military intervention by Russia would mark the beginning of "war."
Meanwhile, the Pentagon said U.S. Defense Secretary Chuck Hagel told his Russian counterpart, Sergei Shoigu, that “Russia risks further instability in the region, isolation in the international community and an escalation that would threaten European and international security."
Addressing an emergency session of the UN Security Council, U.S. Ambassador Samantha Power accused Russia of violating international commitments.
She also called for the "immediate deployment" of OSCE and UN observers to Ukraine to try to defuse the tensions there.
"It is time for the Russian intervention in Ukraine to end. The Russian military must stand down, the aspirations of the Ukrainian people must be respected, and political dialogue must be allowed to continue," Power said.
Ukrainian Ambassador to the UN Yuriy Sergeyev called on the Security Council to “do everything possible” to stop Russian "aggression."
Emergency NATO Meeting
UN Deputy Secretary General Jan Eliasson called for dialogue among all parties.
"It is important to recall the mission of this organization: to always search for peaceful settlement of dispute. This is the essence of the UN Charter and should serve as our primary guide in this situation. Now is the time for cool heads to prevail," Eliasson said.
NATO called for an emergency meeting of its North Atlantic Council, the alliance’s main political decision-making body, to discuss the situation in Ukraine on March 2.
NATO Secretary-General Anders Fogh Rasmussen made the announcement on Twitter, saying: "Urgent need for de-escalation in Crimea."
“Russia must respect Ukraine's sovereignty, territorial integrity, & borders," he added.
Poland's President Bronislaw Komorowski said earlier his country "could feel threatened by a potential Russian military intervention" in neighboring Ukraine.
EU foreign policy chief Catherine Ashton urged Moscow to follow its international obligations to keep forces out of Ukraine and to promote its views through "peaceful means."
The European Union said EU foreign ministers will hold urgent talks on Ukraine on March 3.
The White House said the United States, France, and Canada's leaders "pledged to work together on a package of support and assistance" to Ukraine and also "agreed to continue to coordinate closely, including bilaterally, and through appropriate international organizations."
Both the United States and Canada have suspended their engagements in preparations for the G-8 Summit, currently planned for Sochi.
The Canadian ambassador in Moscow is being recalled for consultations.