NATO called on Russia on Sunday to withdraw its forces from Ukraine to their bases, as Ukrainian officials said they were prepared to defend themselves and US Secretary of State John Kerry prepared a mission to Kiev.
The German government said Russian President Vladimir Putin accepted a proposal by Chancellor Angela Merkel for a fact-finding mission and a "contact group" on Ukraine that could be led by the Organization for Security and Cooperation in Europe.
Meanwhile, in a sign of growing international pressure, Secretary of State John Kerry threatened Russia with sanctions and other economic consequences if it did not back down. He will visit Ukraine's capital Tuesday, a senior administration official said.
Ukraine's security council ordered the general staff to immediately put all armed forces on highest alert, the council's secretary Andriy Parubiy announced. The Defence Ministry was ordered to conduct the call-up, potentially of all men up to 40 in a country that still has universal male conscription.
The country also withdrew its Coast Guard vessels from two Crimean ports, in what was taken as a sign that Russia was nearing the completion of its seizure of the peninsula. But hours later, the Ukrainian government said it had fired its Navy chief and opened a treason case against him for failing to defend his Sevastopol headquarters. He was reported to have appeared on television pledging allegiance to pro-Russian authorities in Crimea.
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"If aggravation is going in that way, when the Russian troops are enlarging their quantity with every coming hour, naturally we will ask for military support and other kind of support," Ukraine's UN ambassador Yuriy Sergeyev told CNN.
Russian forces who have already bloodlessly seized Crimea - an isolated Black Sea peninsula where most of the population are ethnic Russian and Moscow has a naval base - tried to disarm the small Ukrainian contingents there on Sunday. Some Ukrainian commanders refused to give up weapons and bases were surrounded.
A senior U.S. administration official said late Sunday that Russia now has complete operational control of the Crimea peninsula, with 6,000 air and naval forces in the region.
Associated Press journalists reported Sunday that hundreds of unidentified gunmen have arrived outside Ukraine's infantry base in Privolnoye in its Crimea region.
The convoy included at least 13 troop vehicles each containing 30 soldiers and four armored vehicles with mounted machine guns, the AP journalists reported. The vehicles — which have Russian license plates — have surrounded the base and are blocking Ukrainian soldiers from entering or leaving it.
Ukrainian soldiers, with clips in their weapons, have positioned a tank at the gate.
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 neighbor - for which he quickly received the unanimous approval of his parliament - brought the prospect of war to a country of 46 million people on the ramparts of central Europe.
Putin told Merkel on Sunday that Russian citizens and Russian-speakers in Ukraine faced an "unflagging'' threat from ultranationalists, and that the measures Moscow has taken were completely fitting given the ""extraordinary situation," the Kremlin said. The German government said Merkel in turn decried the "unacceptable Russian intervention."
Ukrainian Prime Minister Arseny Yatseniuk, leading a pro-European 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".
Foreign Minister Andriy Deshchytsya said he sent a request to NATO to "examine all possibilities to protect the territorial integrity and sovereignty of Ukraine". Ukraine also appealed for help to Britain and the United States, as co-signatories with Moscow to a 1994 accord guaranteeing Ukraine's security after the breakup of the Soviet Union.
Washington and its allies have suspended plans to attend a G8 summit in Sochi, where Putin had just finished staging his $50 billion winter Olympic games. Dow Jones reported a shortage of foreign currencies in Moscow, with the dollar rising 7% since Friday to an all-time high.
"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 a CBS TV interview, Kerry said G8 nations and some other countries are ``prepared to go to the hilt to isolate Russia'' with a ``broad array of options'' available.
"They're prepared to put sanctions in place, they're prepared to isolate Russia economically, the ruble is already going down. Russia has major economic challenges,'' Kerry said, as he also mentioned visa bans, asset freezes and trade isolation as possible steps.
The economic impact was also being felt in Ukraine; the Wall Street Journal reported that the country's largest commercial bank limited cash withdrawals and suspended all lending.
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.
Ukrainian marines were barricaded into a base in Feodosia, a Crimean port. Russia appealed for them to back the "legitimate" - pro-Russian - regional leadership.
Elsewhere on the occupied peninsula, 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.
(Read more: 'Fascists in power' in Ukraine: Russian officials)
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 authorization to use force in Ukraine would last "until the normalization 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.
In Russia, state controlled media have portrayed Yanukovich's removal as a coup by dangerous terrorists and extremists, funded by the West.
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.
(For more from NBC News on the Ukraine crisis, click here)