Lawmakers in Crimea have voted in favor of the Ukrainian region becoming part of Russia.
Thursday's vote by the Moscow-backed Crimean parliament came as European Union leaders gathered for an emergency summit in Brussels to discuss how to pressure Russia to back down from its military incursion into Crimea.
The government of Crimea, a Black Sea peninsula with a majority Russian population, said it is scheduling a referendum on whether to join Russia for March 16.
Ukrainian Prime Minister Arseniy Yatsenyuk, who was in Brussels for the EU meeting, called the Crimean parliament's vote to join Russia "an illegal decision" and said his government is urging Moscow "not to support those who advocate separatism."
But Yatsenyuk said his government is open to talks about the crisis with Moscow.
"We are not an anti-Russian government; we are pro-Ukrainian," he said. "If they are ready to talk, we are."
Meanwhile, reports from Crimea say a team of observers from the Organization for Security and Cooperation in Europe has been stopped at the border and not allowed into the region. It is not clear whether they were detained.
The White House announced Thursday that is imposing visa restrictions on Russians and Crimeans who threaten Ukraine, and President Barack Obama has authorized sanctions on groups and individuals "responsible for activities undermining democratic processes or institutions in Ukraine."
Earlier, the European Union froze the assets in Europe held by 18 Ukrainians, including ousted President Viktor Yanukovych, former premier Mykola Azarov and 16 former ministers, businessmen and security chiefs.
U.S. lawmakers are also meeting Thursday to discuss potential economic sanctions against Russia.
Meanwhile, direct efforts to end the crisis continue, with U.S. Secretary of State John Kerry meeting again with Russian Foreign Minister Sergei Lavrov in Rome. But Lavrov said after the meeting there is still no agreement between Moscow and Washington.
Kerry held a series of discussions Wednesday with Lavrov, British Foreign Secretary William Hague and Ukrainian interim Foreign Minister Andriy Deshchytsia. The Russian and Ukrainian ministers did not meet face-to-face during Wednesday's flurry of negotiations, and Kerry said he had not expected they would.
Ukraine's crisis began when protests erupted in late November after then-President Viktor Yanukovych rejected a deal with the European Union in favor of closer ties with Russia. What began as peaceful demonstrations eventually turned deadly as protesters clashed with police. Yanukovych fled the country last month.
Concerns the turmoil could spread beyond Crimea remained heightened Thursday, amid clashes between pro-Russian demonstrators and Ukrainian police in the eastern Ukrainian city of Donetsk. Police took back a government building that had been seized Wednesday by demonstrators. Dozens of people were detained.
Western leaders have suggested the crisis in Crimea could be resolved if Moscow pulls back its forces to their bases on the Black Sea and allows in international monitors.