Ukraine has called up its military reservists in response to the Russian parliament's approval of the deployment of troops to Crimea.
Andriy Paruby, Ukraine's top security official, said Sunday the mobilization was initiated to "ensure the security and territorial integrity of Ukraine."
NATO Secretary General Anders Fogh Ramussen said NATO supports the right of the Ukrainian people to determine their future. He said Sunday Ukraine is a "valued" NATO partner and urged Russia to "de-escalate" the tension in the region.
U.S. President Barack Obama held a 90-minute telephone conversation Saturday with Russian President Vladimir Putin to express what the White House calls deep concern for what is happening in Ukraine.
Officials say Mr. Obama told the Russian president that Russia is in clear violation of Ukrainian sovereignty and international law.
President Obama said the United States is suspending participation in meetings to prepare for the G8 economic summit later this year in Sochi, Russia.
Russian news agencies say Mr. Putin told the president that Moscow reserves the right to protect ethnic Russians if there is violence in Crimea or eastern Ukraine.
Mr. Obama said the appropriate way to address this matter is by direct engagement with the Ukrainian government and through international monitors.
Earlier Saturday, Secretary of State John Kerry telephoned interim Ukrainian President Oleksandr Turchynov. Kerry said he commends the Ukrainian government for showing the utmost restraint as it faces a clear and present danger.
The Pentagon says Defense Secretary Chuck Hagel spoke with Russian Defense Minister Sergei Shoygu Saturday. Officials say Hagel told Shoygu that without a change on the ground, Russia is risking more regional instability, global isolation, and an escalation that would threaten European and international security.
NATO, meanwhile announced its ambassadors will meet Sunday in Brussels to discuss Ukraine. A meeting of the NATO-Ukraine Commission is also scheduled.
In New York, the United Nations said now is the time for "cool heads to prevail."
Ukrainian Ambassador Yuriy Sergeyev told the Security Council that 15,000 Russian troops are already in Crimea under the pretense of protecting Russian citizens.
Russian Ambassador Vitaly Churkin blamed the West for ratcheting up tensions in Ukraine and backing protests that ousted President Viktor Yanukovych last month.
The U.S. ambassador to the U.N., Samantha Power, said Russian actions speak louder than words. She said a Russian force in Ukraine could push the situation beyond the breaking point and again called for international mediation in Crimea.
Russia has said its troop movements in Crimea, where it leases a naval base in Sevastopol, conform to agreements with Ukraine.
Crimea is a Black Sea peninsula placed under Ukrainian control in 1954 by then-Soviet leader Nikita Khrushchev. It became part of Ukraine when the Soviet Union collapsed in 1991. Crimea has a tiny border with Russia on its far eastern point. Most of the people living in Crimea are ethnic Russians, but the region also is home to ethnic Muslim Tartars who generally show disdain toward Russia.
Ukraine's troubles began in November when ousted president Yanukovych backed out of a trade deal with the European Union in favor of closer ties and economic aid from Russia.