Russian paramilitaries stand guard outside of a Ukrainian military base in the town of Perevevalne near the Crimean city of Simferopol on March 11, 2014 in Perevevalne, Ukraine.

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.

(Read more: Obama: Sanctionsshow consequence of Ukraine actions)

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.

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.

(Read more: Latest developments in Crimea crisis)



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.