SIMFEROPOL, Ukraine (AFP) - Ukraine stood in danger of breaking apart on Friday after Crimea's parliament unanimously voted to join Russia in a sharp escalation of the worst East-West security crisis since the Cold War.
Both Kiev's new Western-backed interim leaders and US President Barack Obama denounced the decision as illegal and illegitimate.
"If this violation of international law continues, the resolve of the United States and our allies and the international community will remain firm," Mr Obama said after imposing targeted visa bans and setting the stage for wider sanctions against Russia.
The White House said Mr Obama then telephoned Russian President Vladimir Putin to emphasise "that Russia's actions are in violation of Ukraine's sovereignty and territorial integrity, which has led us to take several steps in response".
The Kremlin said Mr Putin tried to calm tensions by saying US-Russian relations "should not be sacrificed due to disagreements over individual - albeit extremely significant - international problems".
The European Union earlier firmed its resolve to impose stiff sanctions on Russia while also vowing to sign a historic trade pact aimed at pulling Kiev out of Moscow's orbit before Ukraine holds snap presidential polls on May 25.
Yet with Moscow's forces in effective control of Crimea - a predominantly ethnic Russian region roughly the size of Belgium - the threat of Ukraine's division seemed more real than at any point since Mr Putin won authorisation from senators last weekend to use military force against his western neighbour.
The scale of that danger intensified further when the city council of the port of Sevastopol, where Moscow's Black Sea Fleet is based, resolved to become "a subject of the Russian Federation" with immediate effect.
Crimea is due to hold a local referendum on March 16 on switching to Russian rule.
The new leaders in Kiev - swept to power on the back of three months of protests against Kremlin-backed leaders that officials said left 100 people dead - immediately took steps to disband Crimea's parliament.
Ukraine's interim premier also appealed for EU powers and the United States to rise to his nation's defence.
"We still believe we can solve in peaceful manner but in case of further escalation and military intervention into Ukraine territory by foreign forces, Ukranian government and military will act in accordance with the constitution and laws," Mr Arseniy Yatsenyuk said in Brussels.
"We are ready to protect our country." Mr Yatsenyuk conceded that Ukraine's forces were dwarfed by the Russian army but said his country's troops had the "spirit" to defend themselves.
Nato chief Anders Fogh Rasmussen earlier said the Western military alliance would review its ties with Russia while boosting cooperation with Ukraine.
Washington announced visa bans on targeted Russians and Ukrainians in the latest in a series of moves by the US administration to punish Moscow for what the White House denounced as "Russia's ongoing violation of Ukraine's sovereignty and territorial integrity".
Mr Obama also authorised freezing the assets of officials involved in ordering Russia's military manoeuvres in Crimea.
Oil prices in New York shot up by almost US$2 (S$2.52 million) a barrel in response to Mr Obama's direct challenge to Moscow.
European leaders - split between hawkish eastern European states and big western European powers that want to limit the damage to their economic relations with Russia because they rely on its natural gas - renewed a commitment to sign an EU association accord with Ukraine by May.
Ousted Ukrainian president Viktor Yanukovych's decision to ditch that pact in November in favour of closer ties with Russia sparked the initial wave of protests that led to his regime's downfall and the rise of the new pro-EU government.
The EU agreed after about six hours of tense discussions to suspend visa and economic talks with Russia - a blow for Moscow's years-long efforts to win open European travel rights.
EU leader also froze the assets of Mr Yanukovych - now living in Russia - and his prime minister Mykola Azarov along with 16 other former ministers.
And they adopted a tough statement demanding Russia enter into negotiations in the next few days to produce "results" on cooling the crisis - threatening travel bans, asset freezes and the cancellation of an EU-Russia summit in June if not.
Russia's EU envoy Vladimir Chizhov dismissed the measures.
"If anyone thinks that Russia can be frightened by such scare tactics, they are deeply mistaken," he told the Interfax news agency.
The epicentre of the crisis has been Crimea: a rugged region seized by Russia in the 18th century and annexed to Ukraine by Soviet leader Nikita Khrushchev as a "gift" in 1954.
Russian forces have surrounded Ukrainian military bases in Crimea since last weekend and on Thursday even scuttled an old warship to block the entrance to a lagoon.
Nervous soldiers stood their ground inside besieged Ukrainian bases. But they seemed ill-prepared to hold off an all-out Russian assault if it came.
"It's extremely tense and I consider it a miracle that bloodshed has been avoided so far," said the Crimea envoy from the Organisation for Security and Co-operation in Europe, Tim Guldimann, after returning from the peninsula.
Mr Guldimann said he cut short his visit after the United Nations envoy Robert Serry was forced to leave on Wednesday upon being confronted by gunmen.
Violent protests have also broken out in cities in mainly Russian-speaking eastern Ukraine, such as Donetsk, Yanukovych's regional stronghold.
The Donetsk regional administration building has been raided repeatedly since Wednesday by pro-Moscow and pro-Kiev crowds. It flew the Ukrainian flag on Thursday evening after the Russian tricolour put up only hours earlier was taken down.
Mr Putin has condemned the changeover of power in Kiev as "an unconstitutional coup" but also said that a full-scale military intervention in Ukraine would be a "last resort". He denies that Russian soldiers are deployed in Crimea.