Ukraine border guards report all the countries borders, apart from Crimea are stable, after the Russian parliament approved deployment of troops to Crimea.
But Ukraine's top security official, Andriy Paruby, says military reservists have been called up to "ensure the security and territorial integrity of Ukraine."
NATO Secretary General Anders Fogh Rasmussen says the Russian move is threatening peace and stability in Europe and urged Russia to "de-escalate" the tension in the region.
U.S. President Barack Obama talked by telephone Saturday with Russian President Vladimir Putin to express deep concern for what is happening in Ukraine.
Officials say Mr. Obama told Mr. Putin Russia is in clear violation of Ukrainian sovereignty and international law.
Meanwhile, Britain has joined the United States in suspending participation in preparatory meetings 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.
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.