Russian President Vladimir Putin's decree recognising Crimea as a sovereign and independent country has triggered the toughest Western sanctions against Russia since the Cold War.
Washington and the European Union reacted to the result of Sunday's referendum, in favour of Crimea breaking away from the Ukraine, by imposing economic and travel restrictions on key figures involved in the crisis.
The EU meted out travel bans and asset freezes on 21 people in Russia and Crimea. These included Alexander Vitko, the commander of Russia's Black Sea fleet, and Sergei Aksyonov, Crimean's unrecognised Prime Minister. Mr Aksyonov's election in a closed session of the regional parliament is not recognised by Kiev.
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Brussels branded Sunday's poll illegal, and left open the possibility of tougher economic measures when EU leaders meet this week.
Foreign Secretary William Hague said: "I think it is an important statement of European unity and resolve, coinciding with the measures taken by the US."
Mr Hague insisted more names could be added to the list, and the scope for future sanctions would take into account Russia's actions in the coming weeks and its reaction to the referendum results.
"This is not a list that is set in stone," he said. "It is possible to add other figures, depending on how Russia reacts to the referendum in Crimea, which has been a mockery of any real democracy, and how they are reacting to the possibility of discussions and direct negotiations with Ukraine over the coming days."
In a statement, EU foreign ministers who met in Brussels said: "Any further steps by the Russian Federation to destabilise the situation in Ukraine would lead to additional and far-reaching consequences for relations in a broad range of economic areas."
Of the 21 people targeted with EU travel bans and asset freezes, 10 were Russian politicians and eight were Ukrainians, mostly from Crimea. Three military officials were also included. As well as Mr Vitko, the sanctions targeted the commanders of Russia's southern and western military districts, which have forces in Crimea.
Ukraine's turmoil has become Europe's most severe security crisis in years and tensions rocketed when Russian troops seized control of Crimea. Russian troops are also massed near the border of eastern Ukraine, which has a large Russian-speaking population.
Ukraine's acting president Oleksandr Turchynov rose tensions further by calling for the activation of 20,000 military reservists and volunteers and for the mobilisation of another 20,000 in the recently formed national guard.
In the Crimean capital of Simferopol, ethnic Russians applauded the referendum which returned a 97% Yes vote for joining Russia.
The US, EU and Ukraine's new government do not recognise the referendum, which was called hastily after pro-Russia President Viktor Yanukovych was removed following months of increasingly violent protests.
The US claimed, in addition to the poll being illegal, that there were massive anomalies in Sunday's balloting.