As Crimeans celebrate an overwhelming vote to rejoin Russia, the European Union said it will decide on sanctions against Moscow on Monday, while the United States said it will never recognize the outcome of today's referendum in Crimea.
Crimeans on Sunday voted overwhelmingly to join former political master Russia, whose president Vladimir Putin insisted that the poll fell completely in line with the norms of international law.
Thousands of people in Crimea celebrated the result into the night, but in other Ukrainian regions there is anger at the possible loss of Crimea to Russia. There is also fear that Mr Putin may seek to intervene in the east of Ukraine, the BBC reports.
In a telephone call on Sunday, US President Barack Obama Mr Putin that the vote in favour of the Crimean Peninsula rejoining Russia violates the Ukrainian constitution.
Officials say 95.5% of voters in Crimea support joining Russia, after half the votes have been counted in the disputed referendum.
Crimea's acting leader Sergei Aksyonov said he would apply to join Russia on Monday. Russian President Vladimir Putin has said he will respect the Crimean people's wishes.
Mr Obama warned Mr Putin that costs would be imposed on Russia for its actions in coordination with European partners.
It's likely the economic sanctions which would freeze the assets of wealthy Russians close to Mr Putin would come in in the next few days, the BBC reports. However, Mr Obama said he still held out hope for a diplomatic way out if Russia scaled back its military activities in Ukraine.
The European Union said it would decide on sanctions against Moscow, including the possible seizure of the foreign assets of top Kremlin officials and travel bans for senior ministers.
China has called for restraint, saying a political settlement is the only way to resolve the crisis in Ukraine. On Saturday, China abstained from voting on a United Nations Security Council resolution that said the referendum was illegal.
Pro-Russian forces took control of Crimea in February this year, moving in after Ukraine's pro-Moscow president, Viktor Yanukovych, was ousted after street protests.
Ukraine's interim president Oleksandr Turchynov said the results had been "pre-planned by the Kremlin as a formal justification to send in its troops".
Referendum boycotted by some
A delegation from the Crimean Parliament will make a formal application to join the Russian Federation this week.
Crimea's acting leader Sergei Aksyonov said the Russian rouble would come into circulation in Crimea alongside the Ukrainian currency, the hryvnia, as early as next week.
About 1.5 million voted in the referendum, which was called by the region's pro-Russian government. It was boycotted by some groups, including the Crimean Tatars and people loyal to Kiev.
On the ballot paper, voters were asked whether they would like Crimea to rejoin Russia. A second question asks whether Ukraine should return to its status under the 1992 constitution, which would give the region much greater autonomy.
In the Ukrainian capital Kiev - 60 years after Crimea was gifted to Ukraine by Soviet leader Nikita Kruschev - the mood was sombre. Authorities in Kiev say the vote was illegitimate, but with Russian forces firmly in control of the region, there is not much they can do.