The EU and US have joined Ukraine’s government in condemning as “illegal” a move by the Crimea region to set up a referendum to endorse joining Russia.
The EU, meeting in Brussels, threatened “serious consequences” if Russia did not act to de-escalate the crisis.
Crimean MPs 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”.
US President Barack Obama urged President Putin to seek a diplomatic solution to the crisis in Ukraine, when he spoke to the Russian leader by telephone for an hour.
President Obama stressed that Russia’s actions were a violation of Ukrainian sovereignty, a White House statement said.
For his part, Mr Putin said that US-Russian “relations should not be sacrificed due to disagreements over individual, albeit extremely significant, international problems,” the Kremlin said.
It was the two leaders’ second telephone call concerning Ukraine in less than a week.
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 MPs’ 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”.