The U.S. is imposing visa restrictions on Russians and Crimeans who it says are threatening Ukraine and also has blocked the transfer of Ukrainian assets out of the United States.

The White House said Thursday that President Barack Obama has signed an executive order authorizing sanctions on groups and individuals "responsible for activities undermining democratic processes or institutions in Ukraine."

Mr. Obama's order did not name any targeted individuals in the asset freeze. But it covers anyone who misappropriated Ukrainian state funds or is threatening the "sovereignty or territorial integrity" of the new Ukrainian government in Kyiv.

The American leader's order comes as the U.S. Congress is rushing to impose further sanctions against Russia for its takeover of Ukraine's Crimean peninsula and provide loan guarantees for the fledgling Ukrainian government. It is an effort aimed at forcing Russian President Vladimir Putin to withdraw Russian troops from Crimea.

The U.S. action comes on top of the European Union move 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.

The European Union said the 18 targeted individuals are subject to criminal proceedings in connection with the embezzlement of Ukrainian state funds and transferring them outside Ukraine. It said the sanctions would apply for a year to give authorities time to recover the money and return it to the new Ukraine government.