The West is facing its biggest confrontation with Russia since the Cold War and Ukraine has marshalled its forces for defence after Russian President Vladimir Putin declared the right to invade his neighbour.
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 said on Sunday (local time).
Andriy Paruby, Secretary of the Security Council which groups top security and defence chiefs, said 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.
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 (local time).
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.
New Zealand's Foreign Affairs Minister Murray McCully said the situation was "deeply worrying".
He called on parties to exercise restraint and "refrain from any action that could escalate tensions".
"The sovereignty and territorial integrity of Ukraine must be respected and maintained," McCully said.
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".
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 Co-operation in Europe, bodies where Moscow would have a veto.
Washington and its allies have suspended plans to attend a G8 summit in Sochi, where Putin had just finished staging his US$50 billion (NZ$59.55bn) winter Olympic games.
"This is probably the most dangerous situation in Europe since the Soviet invasion of Czechoslovakia in 1968," said a Western official on condition of anonymity. "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.
Igor Mamchev, a Ukrainian navy colonel at a small base near the regional capital Simferopol, told Ukraine's Channel 5 television he had refused to surrender.
"A truck with troops of the Russian Federation, armed with rifles, helmets and bullet-proof vests arrived at our checkpoint and suggested we give up our weapons and accept the protection of the armed forces of the Russian Federation," he said.
"I replied that, as I am a member of the armed forces of Ukraine, under orders of the Ukrainian navy, there could be no discussion of disarmament. In case of any attempt to enter the military base, we will use all means, up to lethal force.
"We are military people, who have given our oath to the people of Ukraine and will carry out our duty until the end."
Elsewhere, the Russian forces appeared to be assuming a lower profile on Sunday after the pro-Moscow Crimean leader announced overnight that the situation was now "normalised".
Russians had vanished from outside a small Ukrainian guard post in the port of Balaclava that they had surrounded with armoured vehicles on Saturday.
The Russian forces had been greeted cheerfully by many, with a wedding party honking its horns and people snapping photos. But there were also some voices of worry.
"No one is attacking us, so why do we need protection - much less armed protection? They are filling up Balaclava with troops," said Olga Karpova, 32.
A barricade in front of the Crimean regional parliament, which the Russians seized on Thursday, had been dismantled. A single armoured vehicle with two soldiers drove through the main square, where people were snapping photos.
Ukrainian news agencies reported that Ukrainian marines were barricaded into a base in Feodosia, another Crimean port. Russia has appealed for Ukrainian detachments on the peninsula to back the "legitimate" - pro-Russian - regional leadership.
Putin asked parliament on Saturday to approve force "in connection with the extraordinary situation in Ukraine, the threat to the lives of citizens of the Russian Federation, our compatriots" and to protect the Black Sea Fleet in Crimea.
Putin said his request for authorisation to use force in Ukraine would last "until the normalisation of the socio-political situation in that country".
His justification - the need to protect Russian citizens - was the same as he used to launch a 2008 invasion of Georgia, where Russian forces seized two breakaway regions and recognised them as independent.
There has been effectively no sign of dissent or second guessing in Russia, where state controlled media have relentlessly portrayed Yanukovich's removal as a coup by terrorists and extremists, funded by the West.
In a statement posted online, the Kremlin said that in his phone call with Obama, Putin "underlined that there are real threats to the life and health of Russian citizens and compatriots on Ukrainian territory". Moscow reserved the right to intervene on behalf of Russian speakers anywhere they were threatened.
FLAGS TORN DOWN
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.
A bigger risk would be conflict spreading to the rest of Ukraine, where the sides could not be easily kept apart. Demonstrations turned violent in eastern cities on Saturday.
Demonstrators flew Russian flags at government buildings in the cities of Kharkiv, Donetsk, Odessa and Dnipropetrovsk.
The worst violence took place in Kharkiv, where scores of people were wounded when thousands of pro-Russian activists, some brandishing axe handles and chains, stormed the regional government headquarters and fought pitched battles with a smaller number of supporters of Ukraine's new authorities.
In Donetsk, Yanukovich's home city, the local government has called for a referendum on the region's status, a move Kiev says is illegal. A pro-Russian "self-defence" unit, which staged a big protest on Saturday, scheduled another for Sunday.
"We do not recognise the authorities in Kiev, they are not legitimate," protest leader Pavel Guberev thundered from a podium in Donetsk on Saturday.
Thousands of followers, holding a giant Russian flag and chanting "Russia, Russia" marched to the government headquarters and replaced the Ukrainian flag with Russia's.