The EU, meeting in Brussels, threatened “serious consequences” if Russia did not act to de-escalate the crisis.
Crimea’s parliament earlier set a date of March 16 for a vote on the referendum.
Russian troops took de facto control of Crimea, whose population is mostly ethnic Russian, in the wake of the fall of Ukraine’s pro-Moscow president.
The Crimean parliament on Thursday said it had decided “to enter into the Russian Federation with the rights of a subject of the Russian Federation”.
It said it had asked Russian President Vladimir Putin “to start the procedure”.
Before the Brussels summit, some EU members – led by Germany – had indicated they preferred mediation with Russia to try to solve the crisis, rather than any stronger measures.
But correspondents say the Crimean parliament move has clearly toughened the line taken by the EU.
In press conferences after the talks, German Chancellor Angela Merkel and EU Council President Herman Van Rompuy both said the Crimean referendum was contrary to the Ukrainian constitution and therefore illegal.
The EU said it was suspending talks with Moscow on easing travel restrictions on Russians entering the EU.
It said that if Russia did not move to de-escalate the situation quickly, it would “decide on additional measures, such as travel bans, asset freezes and the cancellation of the EU-Russia summit”.
The EU statement said that “any further steps by the Russian Federation to destabilize the situation in Ukraine would lead to severe and far-reaching consequences… which will include a broad range of economic areas”.
President Barack Obama said the Crimea referendum would “violate the Ukrainian constitution and international law”.
The US president said there was a way to resolve the crisis with Russia through diplomacy but that “if the violation continues, the resolve of the US and its allies will remain firm”.
Barack Obama praised the “international unity on display at this moment”.
The US had earlier issued visa restrictions on a number of unnamed Ukrainian and Russian officials and individuals “to deny visas to those responsible for, or complicit in, threatening the sovereignty and territorial integrity of Ukraine”.
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