By Steve Holland and Lesley Wroughton
WASHINGTON, March 6 (Reuters) - President Barack Obama on Thursday ordered the freezing of U.S. assets and a ban on travel into the United States of those involved in the Russian military intervention into Ukraine's Crimea region.
Obama signed an executive order aimed at punishing those Russians and Ukrainians responsible for the Russian move into Crimea, which has triggered the worst crisis in U.S.-Russian relations since the end of the Cold War.
The order, the White House said in a statement, is "a flexible tool that will allow us to sanction those who are most directly involved in destabilizing Ukraine, including the military intervention in Crimea, and does not preclude further steps should the situation deteriorate."
One such potential for escalation would be any Russian actions in eastern Ukraine, a senior U.S. official told reporters.
The State Department is also putting in place visa bans on a number of officials and individuals responsible for or complicit in threatening the sovereignty and territorial integrity of Ukraine.
However, Russian President Vladimir Putin is not one of those singled out by the sanctions, a senior administration official said.
"It is an unusual and extraordinary circumstance to sanction a head of state, and we would not begin our designations by doing so," the official said.
The order was announced as U.S. Secretary of State John Kerry began a meeting with Russian Foreign Minister Sergei Lavrov in Rome and came on the heels of the Crimean parliament's vote to join Russia and set a referendum on the issue within ten days.
A second senior U.S. official would not link the sanctions to that development but said Crimea could not make that decision on its own.
"With respect to the referendum that was announced, it is the belief of the United States that decisions about Crimea or any part of Ukraine need to be made with the government in Kiev," the official told reporters in a conference call.
A senior State Department official said the United States had informed the Europeans beforehand about the sanctions.
Obama is attempting to rally global opinion against the Russian move, which Putin says was aimed at protecting ethnic Russians in the Crimea region of southern Ukraine. The intervention followed the ouster last month of Ukraine's pro-Russian president.
The United States wants Russian troops to return to their bases in Crimea and for Moscow to allow international monitors into the region to ensure the human rights of ethnic Russians there are protected.
"We call on Russia to take the opportunity before it to resolve this crisis through direct and immediate dialogue with the government of Ukraine," the White House said.
The Obama order targets any assets held in the United States by "individuals and entities" responsible for the Russian military intervention in Ukraine, threatening its territorial integrity or seeking to assert governmental authority over any part of Ukraine without authorization from the Ukrainian government in Kiev.
The White House also said it is prepared to consider additional steps and sanctions as necessary.