SIMFEROPOL, Ukraine — As the pro-Russian authorities in Crimea pressed ahead on Thursday with measures to break away from Ukraine and become part of Russia, President Obama said that their plans for a referendum would “violate the Ukrainian Constitution and violate international law.” The developments came as the United States and the European Unions moved to provide new support for the national government in Kiev and sought ways to press President Vladimir V. Putin of Russia to de-escalate the crisis in Ukraine. President Obama said in Washington: “Any discussion about the future of Ukraine must include the legitimate government of...