President Barack Obama warned of looming consequences for Russia in a conversation Sunday with Vladimir Putin following this weekend’s referendum in Crimea.

Obama reaffirmed the U.S. refusal to recognize the outcome of the vote in Crimea, warning the Russian president that the U.S. and its European partners “are prepared to impose additional costs on Russia for its actions.” The White House had said it would reserve judgment on sanctions until Monday, after the referendum had come to pass. …

“President Obama reiterated that a diplomatic resolution cannot be achieved while Russian military forces continue their incursions into Ukrainian territory and that the large-scale Russian military exercises on Ukraine’s borders only exacerbate the tension. President Obama said that Secretary Kerry continues to be prepared to work together with Foreign Minister Lavrov and the Ukrainian government to find a diplomatic resolution to the crisis.”