U.S. President Barack Obama on Monday imposed sanctions on 11 Russians and Ukrainians blamed for Russia's military incursion into Crimea, including two top aides to Russian President Vladimir Putin.

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The sanctions were the most visible sign of U.S. anger at Russia's attempt to absorb the Crimea region of southern Ukraine, reflecting the deepest plunge in U.S.-Russian relations since the Cold War.

The U.S. sanctions came in an executive order signed by Obama a day after Sunday's Crimea referendum aimed at allowing Russia to annex the region, a vote that the United States says was illegal and will never be recognized by Washington. Obama was to speak on the sanctions at 10:45 a.m. EDT (1445 GMT).

Obama's order freezes any assets in the United States and bans travel into the country of seven ranking Russian government officials and four individuals identified as Crimea-based separatist leaders.

Ousted Ukrainian President Viktor Yanukovich was among those sanctioned and the United States also reached deep into Putin's inner circle by naming presidential aide Vladislav Surkov and adviser Sergei Glazyev. Russia's deputy prime minister, Dmitri Rogozin, and two state Duma deputies, Leonid Slutsky and Yelena Mizulina also were targeted.

Senior administration officials who briefed reporters on the penalties said they were the most comprehensive sanctions applied to Russia since the end of the Cold War.

A senior official said Obama's order clears the way for sanctions on people associated with the Russian arms industry and targets "the personal wealth of cronies" of the Russian leadership.

Putin himself was not sanctioned. A senior Obama administration official said it would have been a highly unusual step and extraordinary to target a head of state.

The administration announced plans for sanctions earlier this month but had not named the individuals until Monday.

Officials warned more sanctions would follow if Russia proceeds with the formal annexation of Crimea, which officials said they believe Putin may announce in a speech on Tuesday.

The White House said the United States would apply the extra sanctions "in a direct and targeted fashion as events warrant."

The European Union also will impose travel bans and asset freezes on 21 Russian and Ukrainian officials, it was announced Monday.

"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.

A senior Obama administration official said there was "concrete evidence" that some ballots in the Crimea referendum arrived in some Crimean cities pre-marked.