President Barack Obama on Monday imposed sanctions against Russian officials, including advisers to President Vladimir Putin, for their support of Crimea’s vote to secede from Ukraine.

The White House also announced that it is working to identify and target the assets of other individuals who aren’t government officials but are supporting them. The Treasury Department also is imposing sanctions on four Ukrainians, including former President Viktor Yanukovych, a former top Ukrainian presidential adviser and two Crimea-based separatist leaders.

“Today’s actions send a strong message to the Russian government that there are consequences for their actions that violate the sovereignty and territorial integrity of Ukraine, including their actions supporting the illegal referendum for Crimean separation,” the White House said in a statement.

“Today’s actions also serve as notice to Russia that unless it abides by its international obligations and returns its military forces to their original bases and respects Ukraine’s sovereignty and territorial integrity, the United States is prepared to take additional steps to impose further political and economic costs,” the statement said.

The U.S. announcement came shortly after the European Union announced travel bans and asset freezes on 21 people they have linked to the unrest in Crimea.

The sanctions were expected after residents in Crimea voted overwhelmingly Sunday in favor of the split. Crimea’s parliament on Monday declared the region an independent state.

The United States, European Union and others say the action violates the Ukrainian constitution and international law and took place in the strategic peninsula under duress of Russian military intervention. Putin maintained that the vote was legal and consistent with the right of self-determination, according to the Kremlin.

Obama’s order targets:

— Vladislav Surkov, a Putin aide

— Sergey Glazyev, a Putin adviser

— Leonid Slutsky, a state Duma deputy

— Andrei Klishas, member of the Council of Federation of the Federal Assembly of the Russian Federation

— Valentina Matviyenko, head of the Federation Council

— Dmitry Rogozin, deputy prime minister of the Russian Federation.

— Yelena Mizulina, a state Duma deputy

(AP)