The NATO head said Saturday that members of the Western military alliance were coordinating their responses to possible Russian military engagement in Ukraine.
Shortly after the Russian Parliament approved the use of military force in Crimea, a western Ukraine region with a large ethnic Russian population, Secretary General AndersFogh Rasmussen urged restraint by the Kremlin.
"Urgent need for de-escalation in Crimea," Rasmussen said in a tweet.
Rasmussen, a former prime minister of Denmark, called on Moscow to recognize the sovereignty of Ukraine, a former Soviet republic that became independent during the collapse of the Soviet Union in 1991.
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"Russia must respect Ukraine's sovereignty, territorial integrity and borders, including with regard to movement of Russian forces in Ukraine," Rasmussen tweeted.
Rasmussen's demands came as the U.N. Security Council met in New York and President Barack Obama held a long, candid phone talk with Russian President Vladimir Putin.
The Russian Parliament on Saturday approved Putin's request for use of military forces in Ukraine. Large numbers of Russian-speaking men wearing unmarked military fatigues patrolled the streets of Simferopol, the regional capital of Crimea near the Black Sea about 400 miles south of the Russian border.
Russia has 26,000 sailors in its Black Sea Fleet at a naval base in Sevastopol, 50 miles southwest of Simferopol.
In a 90-minute phone conversation, Obama told Putin that the United States condemns his country's military intervention into Ukrainian territory.
"President Obama expressed his deep concern over Russia's clear violation of Ukrainian sovereignty and territorial integrity, which is a breach of international law, including Russia's obligations under the U.N. Charter, and of its 1997 military-basing agreement with Ukraine," the office of White House Press Secretary Jay Carney said in a statement.
Obama acknowledged that Russia has deep historic ties to Ukraine and needs to protect the rights of ethnic Russians there.
"President Obama told President Putin that if Russia has concerns about the treatment of ethnic Russian and minority populations in Ukraine, the appropriate way to address them is peacefully through direct engagement with the government of Ukraine and through the dispatch of international observers under the auspices of the United Nations Security Council or the Organization for Security and Cooperation in Europe," Carney's office said.
Obama's national security team met and discussed potential policy options for Ukraine, a senior administration official said.
Obama held separate talks about the Ukraine crisis with the leaders of France and Canada.
Defense Secretary Chuck Hagel delivered a stern message to Russian Defense Minister Sergey Shoygu.
"Secretary Hagel stressed that, without a change on the ground, Russia risks further instability in the region, isolation in the international community and an escalation that would threaten European and international security," Rear Adm. John Kirby, the Pentagon press secretary, said in a statement.
In New York, the U.N. Security Council held an emergency session Saturday afternoon.
U.N. Secretary-General Ban Ki-moon was following events in Ukraine and felt "gravely concerned about the deterioration of the situation," his spokesman said.
"The secretary-general reiterates his call for the full respect for and preservation of the independence, sovereignty and territorial integrity of Ukraine," spokesman Martin Nesirky said in a statement.
In Brussels, NATO ambassadors were scheduled to meet Sunday afternoon.
"NATO allies continue to coordinate closely," Rasmussen said.
The 28-member NATO alliance has expanded to include three former Soviet republics -- the Baltic nations of Latvia, Lithuania and Estonia -- along with former Soviet bloc countries Hungary, Poland, Romania, Albania and Bulgaria.
Four nations that were formerly regions of Soviet bloc nations -- Croatia, Slovenia, Slovakia and the Czech Republic -- also now belong to NATO as independent countries.