President Obama is warning Russian President Vladimir Putin that the U.S. is ready to impose sanctions on Moscow over the Crimea referendum. 

The world leaders spoke by phone today and Obama told Putin the U.S. rejected the results of the vote that would put the Ukrainian region under Russian control.

The White House says Obama "emphasized that Russia's actions were in violation of Ukraine's sovereignty and territorial integrity" and along with European partners, the U.S. is prepared to impose "additional costs on Russia for its actions." 

Obama told Putin the crisis could still be resolved diplomatically but Russia would first need to stop military incursions into Ukraine.