Crimea formally applied to join Russia on Monday after its leaders declared a Soviet-style 97% vote in favour of quitting Ukraine in a referendum condemned as illegal by Kiev and the West that will trigger immediate sanctions.
European Union foreign ministers met in Brussels to decide on visa bans and asset freezes against Russian and Crimean officials held responsible for Moscow's military seizure of the southern Ukrainian region that is home to its Black Sea fleet.
As state media in Russia carried a startling reminder of its power to turn the United States to "radioactive ash", President Barack Obama spoke to Vladimir Putin, telling the Russian president that he and his European allies were ready to impose "additional costs" on Moscow for violating Ukraine's territory.
The Kremlin and the White House issued statements saying Obama and Putin saw diplomatic options to resolve what is the gravest crisis in East-West relations since the Cold War.
But Obama said Russian forces must first end "incursions" into its ex-Soviet neighbour while Putin renewed his accusation that the new leadership in Kiev, brought to power by an uprising that toppled his elected Ukrainian ally last month, were failing to protect Russian-speakers from violent Ukrainian nationalists.
A complete preliminary count of Sunday's vote showed that 96.77% of voters opted to join Russia, the chairman of the regional government commission overseeing the referendum, Mikhail Malyshev, announced live on television.
Officials said the turnout was 83%. Crimea is home to 2 million people. Members of the ethnic Ukrainian and Muslim Tatar minorities had said they would boycott the poll, held just three weeks after Russian forces took control of the peninsula.
Putin, whose popularity at home has been boosted by his action on Crimea despite a big hit to Russian markets, is to address an extraordinary joint session of the Russian parliament about Crimea on Tuesday, Itar-Tass news agency reported. Moscow defended the takeover of the majority ethnic Russian Crimea by citing a right to protect "peaceful citizens".
Ukraine's interim government has mobilised troops to defend against an invasion of its eastern mainland, where pro-Russian protesters have been involved in deadly clashes in recent days.
The Ukrainian parliament on Monday endorsed a presidential decree for a partial military mobilisation to call up 40,000 reservists to counter Russia' military actions.
Military action unlikely
Russia's lower house of parliament will pass legislation allowing Crimea to join Russia "in the very near future", news agency Interfax cited its deputy speaker as saying on Monday.
"Results of the referendum in Crimea clearly showed that residents of Crimea see their future only as part of Russia," Sergei Neverov was quoted as saying.
Japan on Monday echoed Western nations in rejecting the referendum and called on Russia not to annex Crimea. China, which abstained on a UN resolution last week rather than join Russia in vetoing it, called for calm and restraint but avoided comment on the referendum or the annexation issue.
US and European officials say military action is unlikely over Crimea, which Soviet rulers handed to Ukraine 60 years ago. But the risk of a wider Russian incursion, as Putin calculates the West will not respond as he tries to restore Moscow's hold over its old Soviet empire, leaves NATO wondering how to help Kiev without triggering what some Ukrainians call "World War Three".
For now, the West's main tools appear to be escalating economic sanctions, which could seriously weaken the stagnant Russian economy, and diplomatic isolation. China has said it does not back sanctions on Moscow -normally a close diplomatic ally and key economic partner.
Highlighting the stakes, journalist Dmitry Kiselyov, who is close to the Kremlin, stood before an image of a mushroom cloud on his weekly TV show to issue a stark warning. He said: "Russia is the only country in the world that is realistically capable of turning the United States into radioactive ash."
On Lenin Square in the centre of the Crimean capital Simferopol, a band struck up even before polls closed as the crowd waved Russian flags. Regional premier Sergei Aksyonov, a businessman nicknamed "Goblin" who took power when Russian forces moved in two weeks ago, thanked Moscow for its support.
The regional rubber-stamped a plan to transfer allegiance to Russia on Monday before Aksyonov travels to Moscow, although the timing of any final annexation is in doubt. Putin may hold off a formal move as diplomatic bargaining continues over economic and diplomatic sanctions that many EU states fear could hurt them as much as they do Russia.
"Cherish Putin, he is a great, great president!" said Olga Pelikova, 52, as fireworks lit up the night sky and fellow Crimeans said they hoped to share in Russia's oil-fuelled wealth after two decades of instability and corruption in Ukraine.
But many ethnic Tatars, Muslims who make up 12 % of Crimea's population, boycotted the vote, fearful of a revival of the persecution they suffered for centuries under Moscow's rule. "This is my land. This is the land of my ancestors. Who asked me if I want it or not?" said Shevkaye Assanova, a Tatar in her 40s. "I don't recognise this at all."
A pressing concern for the governments in Kiev and Moscow is the transfer of control of Ukrainian military bases. Many are surrounded by and under control of Russian forces, even though Moscow formally denies it has troops in the territory beyond facilities it leases for its important Black Sea Fleet.
On Sunday, the Ukrainian and Russian militaries agreed on a truce in Crimea until March 21, Ukraine's government said. Crimea's parliamentary speaker said on Monday Ukrainian military units in the region would be disbanded although personnel would be allowed to remain on the Black Sea peninsula, Russian news agency Interfax reported.
In Brussels EU foreign ministers were debating the size of a first list of Russian and Crimean officials to be sanctioned individually for actions that "threaten the territorial integrity, sovereignty and independence of Ukraine".
Czech Foreign Minister Lubomir Zaoralek said an initial list of 120 to 130 names had been whittled down to about 20 people. EU leaders meeting on Thursday and Friday may take further measures if diplomacy has not achieved results by then, diplomats said.
The ministers were also expected to cancel an EU-Russia summit scheduled for June in Sochi, where Putin last month hosted the Winter Olympics.
The US administration is also preparing to identify Russians to punish with visa bans and asset freezes that Obama authorised this month. It, too, is likely to act on Monday.