President Barack Obama on Thursday ordered sanctions on people responsible for Moscow's military intervention in Ukraine's Crimea Peninsula, including travel bans and freezing of their U.S. assets, and said a referendum by the region to join Russia would violate international law.
U.S. officials said a list of people targeted by the sanctions had not yet been drawn up, but that Russian President Vladimir Putin was not going to be one of them.
White House spokesman Jay Carney said: "I'm not aware of a limit" on how many people could be listed.
In a separate demonstration of support for Ukraine, the U.S. House of Representatives overwhelmingly passed a bill on Thursday backing loan guarantees for the new government in Kiev. The U.S. Senate is expected to consider a similar bill backing $1 billion in loan guarantees next week.
Obama signed an executive order aimed at punishing those Russians and Ukrainians responsible for the Russian military incursion into Ukraine's Crimea region, which has triggered the worst crisis in U.S.-Russian relations since the end of the Cold War.
Escalating the crisis, Crimea's parliament on Thursday voted to join Russia and its Moscow-backed government set a referendum on the decision in 10 days' time.
Obama, appearing in the White House press room hours after signing the order, said the U.S. sanctions were meant to impose costs on Russia for its actions. He said the international community was acting together and warned that a referendum in Crimea would violate international law as well as the Ukrainian constitution.
"Any discussion about the future of Ukraine must include the legitimate government of Ukraine," Obama said. "In 2014, we are well beyond the days when borders can be redrawn over the heads of democratic leaders."
Obama and administration officials emphasized that the U.S. sanctions could be adjusted or additional steps taken as Russian behavior changed.
"While we take these steps, I want to be clear that there is also a way to resolve this crisis that respects the interests of the Russian Federation, as well as the Ukrainian people," the president said, calling for international monitors to be allowed into Ukraine as well as talks to be held between Moscow and Kiev.
"Russia would maintain its (military) basing rights in Crimea, provided that it abides by its agreements and respects Ukraine's sovereignty and territorial integrity. And the world should support the people of Ukraine as they move to elections in May," he said, calling that the "path to de-escalation."
The White House called the order a "flexible tool" aimed at those directly involved in destabilizing Ukraine, noting that additional steps could be taken if necessary. Any Russian actions in eastern Ukraine would be a potential reason for further measures, a senior U.S. official said.
The State Department is also putting visa bans in place on a number of officials and individuals responsible for, or complicit in, threatening the sovereignty and territorial integrity of Ukraine.
But Putin is not one of those to be singled out, 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.
LEAVING ROOM FOR DIALOGUE
The Crimean Peninsula in the Black Sea has an ethnic Russian majority and is home to a Russian naval base in Sevastopol.
Obama is attempting to rally global opinion against the Russian move, which Putin says was aimed at protecting ethnic Russians in Crimea. The intervention followed the ouster of Ukraine's pro-Russian president last month.
Several hundred chanting demonstrators, many waving blue-and-yellow Ukrainian flags, gathered outside the White House on Thursday to protest Russia's intervention in Crimea.
They carried signs that read: "Putin Sucks," and "Putin is a war criminal," and chanted: "Russia, hands off Ukraine!" and in Ukrainian: "Glory to Ukraine, glory to the heroes of Ukraine!"
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.
Obama's order was announced as U.S. Secretary of State John Kerry began a meeting with Russian Foreign Minister Sergei Lavrov in Rome. Kerry, speaking in Rome, noted that the sanctions framework was designed to allow talks to go forward.
"We want to be able to continue the intense discussions with both sides in order to try to normalize and end this crisis," he said. "We will absolutely consider if we have to additional steps beyond what we've done, but our preference ... is to emphasize the possibilities for the dialogue that can lead to the normalization and defusing of this crisis."
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 government in Kiev.
A senior State Department official said the United States informed the Europeans beforehand about the sanctions.
Additional reporting by Mark Felsenthal, Patricia Zengerle and Lacey Johnson in Washington, and Lesley Wroughton in Rome; editing by Mohammad Zargham, Peter Cooney and G Crosse