Obama announces sanctions against Russian officials after Crimea vote

"Continued Russian military intervention in Ukraine will only deepen Russia's diplomatic isolation," he said.

The new penalties build on an executive order from the White House earlier this month that imposed visa restrictions on pro-Russian opponents of the new government in Ukraine, bringing the total number of individuals facing sanctions to 11. Senior administration officials who briefed reporters on the order described the targeted individuals as "cronies" of the Russian government and arms industry who had had taken a leading role in envisioning or implementing the referendum strategy, including former Ukrainian President Viktor Yanukovych.

"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. A senior administration official later called the action, "far the most comprehensive sanctions applied to Russia since the end of the Cold War."

The European Union also announced Monday it would freeze assets of and ban travel for 21 officials in Russia and Ukraine. Although that list has not yet been released, a senior administration official indicated there would be some overlap between the two groups.

"We urge our counterparts and financial institutions around the world to shun these individuals," a senior administration official said.

During a briefing later in the day, White House Press Secretary Jay Carney refused to rule out the possibility that Russian President Vladimir Putin might be the target of future sanctions.

"We're not going to rule out individuals or rule out actions except to say that there will be costs imposed on Russia, additional costs opposed on Russia if Russia does not change direction here," he said.

Although the White House is using the sanctions to punish Russia, it is still seeking a diplomatic resolution to the conflict in Ukraine. After the results of the referendum Sunday - in which 97 percent of voters backed secession and an ultimate reunification with Russia - Mr. Obama spoke with Putin to reprimand him for supporting the referendum and say there could be no resolution so long as Russian troops continued to remain inside Ukraine and conduct military exercises on the border.

The president said Monday that that the U.S. would "calibrate" its response based on whether Russia chooses to escalate or deescalate the situation.

Mr. Putin has shown no willingness to back down, however, and is scheduled to address the Russian Federal Assembly Tuesday and is widely expected to recommend formal annexation of Crimea, according to the administration.

The Crimean Peninsula had been a part of Russia until Nikita Khrushchev, the leader of the U.S.S.R., gave it to the Ukrainians in 1954 to mark the 300th anniversary of Russia's acquisition of Ukraine in the Treaty of Pereyaslav.

Vice President Biden is set to travel to Poland and Lithuania this week to discuss the situation in Ukraine with officials there.