Crimea's parliament voted to join Russia on Thursday and its Moscow-backed government set a referendum on the decision in 10 days' time in a dramatic escalation of the crisis over the Ukrainian Black Sea peninsula.
US President Barack Obama led European Union and Ukrainian criticism of the proposed vote, saying it would violate international law.
The sudden acceleration of moves to bring Crimea, which has an ethnic Russian majority and has effectively been seized by Russian forces, formally under Moscow's rule came as European Union leaders held an emergency summit groping for ways to pressure Russia to back down and accept mediation.
The 28-nation EU condemned Russian actions in Crimea as illegal, voiced support for Ukraine's territorial integrity but took only minor steps suspending talks with Moscow on visas and a new investment pact while warning of tougher steps if there is no negotiated solution within a short period.
In a signal to Moscow, Obama announced plans to punish Russians and Ukrainians involved in what he called "threatening the sovereignty and territorial integrity of Ukraine", ordering the freezing of their US assets and a ban on travel to the United States. A US official said Russian President Vladimir Putin was not on the list.
"The proposed referendum on the future of Crimea would violate the Ukrainian constitution and violate international law," Obama told reporters at the White House.
"Any discussion about the future of Ukraine must include the legitimate government of Ukraine," he said.