Ukraine on Friday faced the threat of breaking apart after Crimea's parliament voted to join Russia in a sharp escalation of the worst East-West security crisis since the Cold War.
The powerful speaker of Russia's house of parliament said Moscow intended to respect Crimea's "historic choice," which both Kiev's new Western-backed interim leaders and US President Barack Obama denounced as illegal.
"If this violation of international law continues, the resolve of the United States and our allies and the international community will remain firm," Obama said after imposing targeted visa bans and setting the stage for wider sanctions against Russia.
The White House said Obama then telephoned Russian President Vladimir Putin for an hour to emphasise "that Russia's actions are in violation of Ukraine's sovereignty and territorial integrity."
The Kremlin said Putin tried to calm tensions by stressing that 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 an historic trade pact aimed at pulling Kiev out of Moscow's orbit before Ukraine holds snap presidential polls on May 25.
Yet with Russian forces in effective control of Crimea -- a predominantly ethnic Russian peninsula roughly the size of Belgium and base of Kremlin's Black Sea fleet -- the threat of Ukraine's division seemed more real than at any point since Putin got parliamentary approval to use force against ex-Soviet Ukraine.
Western allies have been grappling with a response to Putin's seeming ambition to recreate vestiges of the Russian empire without regard to the damage this does to Moscow's foreign relations or instability it creates.
Moscow argues it needs to defend ethnic Russians from coming under attack from ultra-nationalists who have backing from the new pro-EU team in Kiev.
Putin has previously denounced the interim leaders' rise to power as an "unconstitutional coup".
The tensions in Ukraine intensified still further when the city council of Sevastopol that houses the Kremlin's Black Sea Fleet also 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 -- a decision welcomed Friday by Russia's State Duma speaker Sergei Naryshkin.
"We will respect the historic choice of the people of Crimea," said the close Putin ally. "We support the free and democratic choice of the population of Crimea."
The new leaders in Kiev -- swept to power on the back of three months of protests against a Kremlin-backed regime that left 100 people dead -- immediately took steps to disband Crimea's parliament.
Ukraine's interim prime minister Arseniy Yatsenyuk also appealed for EU powers and the United States to rise to his nation's defence.
"I would like to say to Mr Putin: Tear down this wall, the wall of intimidation, of military aggression," Yatsenyuk said in an echo of former US President Ronald Reagan's 1987 address at the Berlin Wall.
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".
Obama also authorised freezing the assets of officials involved in ordering Russia's military manoeuvres in Crimea.
European leaders -- split between hawkish eastern European states many of which were under Kremlin's zone of influence during the Cold War and big western European powers that want to limit the damage to their economic relations with Russia -- 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 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.
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 and asset freezes along with the cancellation of an EU-Russia summit in June if not.
EU leaders also froze the assets of Yanukovych -- now living in Russia -- and his prime minister Mykola Azarov along with 16 other former ministers.
Interpol said Friday it was considering a Ukrainian government request to issue an arrest warrant for the deposed head of state.
The epicentre of the crisis has been Crimea, a rugged Black Sea peninsula seized by Russia in the 18th century and annexed to Ukraine by Soviet leader Nikita Khrushchev as a "gift" in 1954.
Obama is pushing terms of a diplomatic solution that would see Russia call back troops to their barracks and accept international observers from the United Nations and the Organisation for Security and Co-operation in Europe (OSCE).
But pro-Kremlin gunmen on Thursday stopped a team of 40 military OSCE observers from entering Crimea. The military team was expected to try again on Friday.