Having failed to prevent a Russian-sponsored referendum in Crimea, the Obama administration and its European allies refocused their efforts Sunday on keeping Moscow from annexing the autonomous Ukrainian region and expanding its military moves into other parts of Ukraine. In a telephone call to Russian President Vladimir Putin — his third in two weeks — President Obama said that the referendum “would never be recognized by the United States and the international community” and that “we are prepared to impose additional costs on Russia for its actions,” the White House said.