Crimea's parliament speaker Vladimir Konstantinov (3rd L) and Russia's upper house speaker, Federation Council chief Valentina Matviyenko (R) attend a meeting in Moscow on March 7, 2014.
Top Russian lawmakers on Friday welcomed the prospect of Crimea joining the country, despite stiffening sanctions on Moscow over the worst East-West crisis since the Cold War.
The heads of Russia's two houses of parliament said they would respect a decision by lawmakers in Ukraine's majority-Russian peninsula to renounce ties with Kiev and stage a March 16 referendum on switching over to Kremlin rule.
"Should the people of Crimea decide to join Russia in a referendum, we... will unquestionably back this choice," said speaker of the upper house Valentina Matviyenko.
"We will respect the historic choice of the people of Crimea," said her lower house counterpart Sergei Naryshkin.
The escalating threat of the ex-Soviet nation of 46 million splintering between its pro-European west and more Russified southeast prompted US President Barack Obama to place an hour-long call to Russian counterpart Vladimir Putin.
It marked the leaders' second lengthy phone call in five days and both sides described it as tough.
The White House said Obama "emphasised that Russia's actions are in violation of Ukraine's sovereignty and territorial integrity, which has led us to take several steps in response, in coordination with our European partners."
The Kremlin for its part 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 the base of the Kremlin's Black Sea Fleet -- the threat of Ukraine's division seemed more real than at any point since Putin won parliamentary approval to use force against his western neighbour.
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.
The new leaders in Kiev who swept to power on the back of three months of protests against a Kremlin-backed regime that claimed 100 lives, 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.
Yatsenyuk on Friday called Crimea's pro-Kremlin leaders "traitors" and said "no part of Ukraine will ever be a part of Russia".
- US, EU sanction Russia -
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 are split between hawkish eastern European states that 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 who is now living in Russia and his prime minister Mykola Azarov along with 16 other former ministers.
- Observers kept out of Crimea -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.
Ukrainian soldiers said they were under orders from their superiors to avoid a confrontation with the Russians that could potential spark an all-out conflict on Europe's eastern edge.
"Now the orders are to stand guard by the gates for four hours in shifts," said a 27-year-old aviation mechanic named Oleksandr as he stared out from the gates of a base near a Ukrainian military airport in Sevastopol.
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 after spending the night in the Ukrainian city of Kherson near the entrance to the peninsula.
A Western diplomatic source said the observers had been blocked "by two groups of armed people -- very professional, very well-trained".
Violent protests have also broken out in cities in mainly Russian-speaking eastern Ukraine such as Donetsk that have been the strongholds of support.
The Donetsk regional administration building has been raided repeatedly by both pro-Moscow and pro-Kiev crowds. It flew the Ukrainian flag on Friday morning after the Russian tricolour had been put up the day before.