President Barack Obama says new sanctions will "impose a cost" on Russia for its military move into Crimea and that a planned referendum on whether the peninsula should join Russia violates international law and Ukraine's constitution.
Mr. Obama told reporters at the White House an executive order he signed Thursday authorizes sanctions, including visa restrictions, on those who violate Ukraine's territorial integrity or steal the assets of the Ukrainian people.
He said that the steps were taken in "close coordination" with America's European allies, adding that he was "pleased that our international unity is on display at this critical moment."
The president said a referendum in Ukraine's Crimea region set for March 16 on whether to become part of Russia would violate international law. Mr. Obama said that any discussion about Ukraine's future "must include the legitimate government of Ukraine."
"In 2014 we are well beyond the days when borders can be redrawn over the heads of democratic leaders," Mr. Obama said.
Mr. Obama's executive order also blocks the transfer out of the United States of assets belong to anyone who has undermined democratic institutions in Ukraine, misappropriated its economic assets, or threatened its territorial sovereignty.
The order does not name any targeted individuals.
The presidential move comes as the U.S. Congress is rushing to impose further sanctions against Russia for its takeover of Crimea and to provide loan guarantees for the fledgling Ukrainian government.
Earlier, the European Union moved to freeze 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. Included on the list are Mr. Yanukovych's two sons and Azarov's son.
Meanwhile, the U.S. State Department has issued a paper to counter what it says are 10 "false claims" made by Russia's President Putin about Ukraine.
Responding to Mr. Putin's claim that "citizens' defense groups," not Russian forces, have seized Ukrainian infrastructure and military facilities in Crimea, the State Department says there is "strong evidence" that "members of Russian security services are at the heart of the highly organized anti-Ukraine forces in Crimea."