WASHINGTON — 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. They are the most comprehensive sanctions against Russia since the end of the Cold War.

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 – under existing authority under a previous Obama order. Senior administration officials also said they are working to identify what they called “Russian government cronies” to target the assets of those supporting the Crimea unrest, including individuals working in the arms industry.

Obama made a statement Monday from the White House.

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.

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.