The United States on Monday imposed sanctions on seven Russians — including close aides of President Vladimir Putin — and four Ukrainians blamed for Russia's military incursion into Crimea.
The Europe Union also targeted Putin's aides, slapping sanctions on senior officials to pressure the Kremlin to abandon moves to annex Crimea. The EU list has 21 individuals who remained unidentified.
A senior US official described the measures, announced after the breakaway Ukrainian region of Crimea voted to join Russia in a referendum, as the "first comprehensive sanctions (against Russia) since the end of the Cold War". The referendum has been deemed unlawful by Kiev and the West.
Read: Crimea's 6 steps towards joining Russia after Soviet-style vote
A pro-Russian protester celebrates in Simferopol's Lenin Square on March 16, 2014 after exit polls showed that about 95% of voters in Ukraine's Crimea region supported union with Russia (AFP photo)
US President Barack Obama said the sanctions made it clear "that there are consequences for their actions" in Crimea and that the US was ready to impose further sanctions if necessary.
Obama had earlier warned Russia of "costs" of military intervention in Crimea and Ukraine. The US President "broadly repeated" his threat in a phone conversation with Putin on Sunday.
The sanctions will freeze assets held by designated individuals in the US and prohibit American citizens from doing business with them. Officials said such sanctions are known to make it difficult for designated individuals to conduct business not only in the United States but also in Europe and Asia.
The seven Russians named in the sanctions were Vladislav Surkov, Sergey Glazyev, Leonid Slustky, Andreai Klishas, Valentina Matviyenko, Dmitri Rogozin and Yelena Mizulina.
Each of them "played a leading role as an ideologist, as a strategist and as an architect of the referendum strategy and is a leading proponent of formal annexation of Crimea by Russia", a senior US official said.
Surkov and Glaznyev are close aides of Putin and Rogozin is the deputy prime minister. The others include officials of the Russian parliament Duma.
Apart from deposed president Yanukovych, the Ukrainians named by the US are Crimea's prime minister Sergey Aksyonov and speaker of parliament Vladimir Konstantinov. Viktor Medvedchuk, a pro-Russia activist, is also named.
Read: All you need to know about Crimea's referendum
"The steps taken by the separatist leaders designated today and former president Yanukovych violate Ukraine's sovereignty and territorial integrity," US treasury official David Cohen said.
Obama said a diplomatic solution to the crisis was still possible if Russia pulls its troops back to barracks in Crimea, allows foreign observers to deploy and agrees to negotiate with Ukraine.
Read: Crimea holds secession referendum amid turmoil
Pro-Russian protesters celebrate in Simferopol's Lenin Square on March 16, 2014 after exit polls showed that about 95% of voters in Ukraine's Crimea region supported union with Russia (AFP photo)
But he warned: "Further provocations will just further isolate Russia and diminish their place in the world."
US officials said the moves were intended to strike at "cronies" around Russia's President Putin, and to impose a heavy cost for the Crimean referendum and the arrival of Kremlin forces.
European Union foreign ministers unveiled travel bans and asset freezes against 13 Russian officials and eight Ukrainian officials from Crimea.
Putin may face international condemnation for his fast-track seizure of Crimea, but asset freezes and travel bans imposed by the West may not force him to alter course as he rides high in the polls.
Putin has seen his approval rating rise sharply since it became clear that Russia was taking control of Crimea, according to both state and independent polling agencies.
With inputs from Agencies