WASHINGTON (AP) — In the most comprehensive sanctions against Russia since the end of the Cold War, President Barack Obama on Monday froze the U.S. assets of seven Russian officials, including top advisers to President Vladimir Putin, for their support of Crimea's vote to secede from Ukraine.

Obama said he was moving to "increase the cost" to Russia, and he warned that more people could face financial punishment.

"If Russia continues to interfere in Ukraine, we stand ready to impose further sanctions," Obama said. He added in a brief statement from the White House that he still believes there could be a diplomatic resolution to the crisis and that the sanctions can be calibrated based on whether Russia escalates or pulls back in its involvement.

The Treasury Department also is imposing sanctions on four Ukrainians — including former President Viktor Yanukovych and others who have supported Crimea's separation — under existing authority under a previousObama order.

"We are imposing sanctions on specific individuals responsible for undermining the sovereignty, territorial integrity and government of Ukraine. We're making it clear that there are consequences for their actions,"Obama said.

Senior administration officials also said they are developing evidence against individuals in the arms industry and those they described as "Russian government cronies" to target their assets.

The administration officials said Putin wasn't sanctioned despite his support of the Crimean referendum because the U.S. doesn't usually begin with heads of state. But the officials, speaking to reporters on a conference call on the condition they not be quoted by name, say those sanctioned are very close to Putin and that the sanctions are "designed to hit close to home."

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. Obama administration officials say there is some overlap between the U.S. and European list, which wasn't immediately made public. Biden was heading to Europe Monday and Obama plans to go next week. The president said that demonstrating a "solemn commitment to our collective defense" as NATO allies will be at the top of the agenda.

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 administration officials say there is some concrete evidence that some ballots for the referendum arrived pre-marked in many cities and "there are massive anomalies in the vote." The officials did not say what that evidence was.

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.

The administration officials said they will be looking at additional sanctions if Russia moves to annex Crimea or takes other action. Those targeted will have all U.S. assets frozen and no one in the United States can do business with them under Obama's order.

"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.

Administration officials say those Obama targeted also are key political players in Russia also responsible for the country's tightening of human rights and civil liberties in the country. Obama's order targets were:

— 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

The four newly targeted by the Treasury Department are:

— Yanukovych, who fled Ukraine for Russia and has supported the dispatch of Russian troops into Ukraine

— Viktor Medvedchuk, the leader of Crimea separatist group Ukrainian Choice and a close friend of Putin

— Sergey Aksyonov, prime minister of Crimea's regional government

— Vladimir Konstantinov, speaker of the Crimean parliament
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(Photo: AP Photo/Pablo Martinez Monsivais)