Ukraine is calling up all military reserves and must ensure the armed forces are combat-ready as soon as possible, the country's top security official has said.
Andriy Paruby, Secretary of the Security Council which groups top security and defence chiefs, said on Sunday that an order had also been given to the Foreign Ministry to seek US and British help in guaranteeing the security of Ukraine.
The armed forces, he said, would step up security at energy facilities, the Reuters news agency reported.
The West is facing its biggest confrontation with Russia since the Cold War after Russian President Vladimir Putin declared the right to invade his neighbour.
Ethnic Russian forces have already bloodlessly seized Crimea, an isolated Black Sea peninsula where most of the population are Russian and Moscow has a naval base, and sought to disarm the small Ukrainian contingents there on Sunday.
Putin has defied calls from the West to pull back his troops, insisting that Russia has a right to protect its interests and the Russian-speaking population in Crimea and elsewhere in Ukraine.
Of potentially even greater concern are eastern swathes of the country, where most of the ethnic Ukrainians speak Russian as a native language.
Those areas saw violent protests on Saturday, with pro-Moscow demonstrators hoisting flags at government buildings and calling for Russia to defend them.
Putin's declaration that he has the right to invade his neighbour - for which he quickly received the unanimous approval of his parliament - opened the prospect of war in a country of 46 million people on the ramparts of central Europe.
"President Obama expressed his deep concern over Russia's clear violation of Ukrainian sovereignty and territorial integrity, which is a breach of international law," the White House said after the two leaders spoke for 90 minutes.
Ukrainian Prime Minister Arseny Yatseniuk, leading a government that took power after Moscow ally Viktor Yanukovich fled a week ago, said Russian armed action "would be the beginning of war and the end of any relations between Ukraine and Russia".
Acting President Oleksander Turchinov ordered troops to be placed on high combat alert. Foreign Minister Andriy Deshchytsya said he had met European and US officials and sent a request to NATO to "examine all possibilities to protect the territorial integrity and sovereignty of Ukraine".
'Crimea in Russian hands'
NATO ambassadors were due to meet in Brussels on Sunday to discuss the situation. Washington has proposed sending monitors to Ukraine under the flags of the United Nations or Organisation for Security and Cooperation in Europe, bodies where Moscow would have a veto.
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"This is probably the most dangerous situation in Europe since the Soviet invasion of Czechoslovakia in 1968," a Western official told Reuters on condition of anonymity.Washington and its allies have suspended plans to attend a G8 summit in Sochi, where Putin had just finished staging his $50bn winter Olympic games.
"Realistically, we have to assume the Crimea is in Russian hands. The challenge now is to deter Russia from taking over the Russian-speaking east of Ukraine."
In Crimea, Ukraine's tiny military contingent was powerless to oppose Russian forces, who bore no insignia on their uniforms but drove vehicles with Russian plates and seized government buildings, airports and other locations in the past three days.
Russian news agencies reported that Moscow's troops had disarmed Ukrainians at several small bases.
So far there has been no sign of Russian military action in Ukraine outside Crimea, the only part of the country with a majority that is ethnically Russian and which has often voiced separatist aims at times of tension between Moscow and Kiev.