EU foreign ministers are due to discuss further sanctions against Russia after a Moscow-backed referendum in Crimea backed a split from Ukraine.
The 28-member bloc is considering a visa ban and an asset freeze against a number of Russian officials.
More than 95 percent of Crimean voters backed joining Russia, local officials said. Moscow says it will accept the result.
The EU and US say the vote is illegal in Crimea, where pro-Russian forces took control in February.
US President Barack Obama has warned Moscow that Washington is also ready to impose "costs" over its actions in Ukraine.
The new authorities in Kiev say Russian troops moved in to Crimea after Ukraine's pro-Moscow president, Viktor Yanukovych, was ousted following months of street protests.
The EU foreign ministers are expected to gather in Brussels at 08:30 GMT.
The bloc has already suspended talks on an economic pact with Russia and an easing of visa restrictions.
The ministers will now be toughening the measures by discussing a list of names of those who could be subject to an asset freeze and a visa ban.
However, the question remains whether it should just be the list of people related to the takeover in Crimea or should it target the inner circle around Russian President Vladimir Putin, the BBC's Europe editor Gavin Hewitt says.
He adds that - not surprisingly - there are divisions among the 28 nations.
One official said bluntly if you did not go after people in Moscow you were showing a weak hand. Others argued that Brussels could not impose bans on people it would be sooner or later have to negotiate with.
If an asset freeze and visa ban does not rattle Moscow or Russia makes further military moves into eastern Ukraine - then the EU has already warned of moving to another stage which would involve economic sanctions.
Earlier, the EU said in a statement that the vote was "illegal and illegitimate and its outcome will not be recognised".
On Sunday night, thousands of Crimeans began to celebrate as the vote's first results were announced.
Local elections officials later said that with three-quarters of the ballots counted, some 95.7 percent of the voters backed re-joining Russia. The full results are expected later on Monday.
Sergei Aksyonov, Crimea's pro-Moscow leader installed last month after the Russian takeover, appeared on stage in Crimea's regional capital, Simferopol.
Backed by the Russian national anthem, Russian flags, and the personnel of Russia's Black Sea fleet, he told supporters that Crimea was "going home".
Aksyonov said Crimea's parliament, which was disbanded by the government in Kiev last week, would send a formal request to Moscow to join Russia on Monday.
Some 58 percent of people in Crimea are ethnic Russian, with the rest made up of Ukrainians and Tatars.
Most of the Tatars that the BBC spoke to said they had boycotted the vote, and felt that life under the Kremlin would be worse.
Refat Chubarov, leader of the Tatars' unofficial parliament, said the referendum was illegal, and held in a hasty manner under the control of Russian troops.
"The fate of our motherland cannot be decided in such a referendum under the shadows of the guns of soldiers," he told the BBC.
The Tatars were deported to Central Asia by Soviet dictator Joseph Stalin in 1944. They were only able to return with the fall of the Soviet Union and many want to remain in Ukraine.
But the referendum did not have an option for those who wanted the constitutional arrangements to remain unchanged.
Voters were asked whether they wanted to join Russia, or have greater autonomy within Ukraine.