Speaking from the White House, Obama said any decisions on the future of Crimea, a pro-Russian area of Ukraine, must include the country's new government.
"The proposed referendum on the future of Crimea would violate the constitution and violate international law," Obama said. "We are well beyond the days when borders can be redrawn over the heads of democratic leaders."
Obama spoke hours after a March 16 date was set for a referendum on whether the region should become part of Russia.
Russian forces began moving into Crimea about a week ago, despite Obama's warnings that there would be costs for such actions. Seeking to follow through on that threat, Obama moved Thursday to enact new visa restrictions on an unspecified and unidentified number of people and entities that the U.S. accused of threatening Ukraine's sovereignty and territorial borders. The restrictions were unlikely to directly target Russian President Vladimir Putin.
Obama also signed an executive order that will allow the U.S. to levy financial sanctions. In a statement, the White House said the penalties would target "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."
In Brussels, meanwhile, the European Union announced it was suspending talks with Russia on an economic pact and on a visa deal in response to the Russian intervention in Crimea. EU leaders, like Obama, threatened further sanctions if Russia pushes ahead.
"I am confident that we are moving forward together, united in our determination to oppose actions that violate international law and to support the government and people of Ukraine," Obama said.
Senior Obama administration officials said penalties will deepen significantly if Russia presses into areas of eastern Ukraine, though they said there is currently no indication Moscow has taken that step. The officials also indicated that the penalties could be ratcheted down if Russia pulls back its troops in Crimea and recognizes Ukraine's new government.
"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 statement read.
Ukraine's unrest peaked in February, after months of pro-Western protests seeking the overthrow of President Viktor Yanukovych in anger over economic woes and corruption. Yanukovych, who is pro-Russian, fled for protection to a location just outside of Moscow, and Putin sent troops into Crimea in a show of force against the upstart government in Kiev.
Crimea is a peninsula that hosts a major Russian navy base and is historically and culturally a Russian stronghold.
The visa bans will be imposed immediately and come in addition to an earlier State Department decision to deny U.S. entry to those involved in human rights abuses related to political oppression in Ukraine. Officials would not say whether Yanukovych was a target of the visa ban or the sanctions.
The sanctions plan, outlined in an executive order, lays the legal groundwork for the Treasury Department to impose financial penalties on offenders. The aim is clearly to punish the separatist movement in Crimea as well as Russia for its decision to send military forces there.
Specifically, the sanctions would target people who undermine Ukraine's democracy and new government; threaten the country's peace, security, stability and sovereignty; are linked to misappropriations of government assets; and try to assert governmental authority over any part of Ukraine without the consent of Kiev. They would also prohibit U.S. persons from doing business with those who have been sanctioned.
Congress has been rushing to impose hard-hitting sanctions on Russia in response to its takeover of Crimea, hoping Europe will follow the lead of the United States in upping the pressure on Putin's government.
The U.S. sanctions push represents a rare case of broad agreement among the Obama administration and Democrats and Republicans in both houses of Congress. But they all are also united in their concern that American economic penalties will mean little without the participation of European countries with far deeper commercial relations with Russia.
Brendan Buck, a spokesman for House Speaker John Boehner, welcomed the sanctions and said the Ohio Republican is "committed to working with the administration to give President Obama as many tools as needed to put President Putin in check as well as prevent Russia from infringing on the sovereignty of any of its other neighbors."
Rep. Adam Schiff, a California Democrat, called the new penalties "a positive first step" that needs to be coupled with similar sanctions from Europe to underscore the costs - diplomatic and economic - of rejecting Ukraine's sovereignty.
"I hope that Russia can be dissuaded from further aggression and can be walked back from its perilous course," Schiff said. "But if not, the U.S. and its allies must be prepared to use all of the diplomatic and economic tools at its disposal to deter such reckless conduct."
Jakes reported from Rome. Associated Press writer Bradley Klapper in Washington contributed to this report.
US fighter jets, warship arrive in Ukraine region
The Pentagon says six U.S. F-15 fighter jets have arrived in Lithuania to bolster air patrols over the Baltics as the stand-off over Russia's military incursion into Ukraine continues. A U.S. warship is also now in the Black Sea to participate in long-planned exercises.
The fighter jets and 60 U.S. military personnel landed at Siauliai Air Base in Lithuania, adding to the four F-15s and 150 troops already there to do the air patrol mission. The additional fighters came from RAF Lakenheath.
The Navy destroyer USS Truxtun is participating in exercises with Romania and Bulgaria and is expected to be in the Black Sea for several days.
The U.S. and other Western nations are preparing sanctions against Russia for its recent move to send military troops into Ukraine's Crimean Peninsula.
EU slaps initial sanctions on Russia
BRUSSELS (AP) - The European Union suspended talks with Russia on a wide-ranging economic pact and a visa agreement Thursday in response to its military incursion into Ukraine's Crimean Peninsula, threatening tougher sanctions unless Moscow swiftly defuses the crisis.
The moves at an emergency EU summit came on the heels of visa and financial sanctions the Obama administration imposed on Russians and Ukrainians over the military incursion into Crimea.
EU President Herman Van Rompuy said further measures could include travel bans, asset freezes and the cancellation of an EU-Russia summit if Moscow does not quickly end its aggression and joins meaningful, multilateral talks within days to halt the crisis.
"We are in close coordination with the United States on this," German Chancellor Angela Merkel said. "We cannot go back to business as usual" with Russia, she added.
However, the EU's latest sanctions appeared weak compared to the U.S. ones and to what some more hawkish EU countries wanted, particularly those bordering Russia. Poland's leader noted the resistance to penalizing Moscow remains fairly high among some members of the 28-nation bloc because of Europe's close proximity, energy dependence and trade ties to Russia.
As the EU leaders met, the U.S. also sent six F-15 fighter jets to Lithuania to bolster air patrols over the Baltics, and a U.S. warship is now in the Black Sea to participate in long-planned exercises.
The sanctions on both sides of the Atlantic aimed to rein in Europe's gravest geopolitical crisis in a generation, which developed swiftly again Thursday with Crimean lawmakers declaring their intention to split from Ukraine and join Russia instead and scheduling a referendum in 10 days for voters to decide the fate of the disputed peninsula.
Visiting the summit, Ukraine's Prime Minister Arseniy Yatsenyuk branded the referendum illegitimate. "Crimea was, is, and will be an integral part of Ukraine," he told reporters.
In Washington, President Barack Obama said the referendum would violate international law.
The EU put on ice talks on a wide-ranging economic agreement and on granting Russian citizens visa-free travel within the 28-nation bloc, a goal that Moscow has been pursuing for years.
The decision followed tough negotiations among member states divided over how to react to the Russian aggression.
"Not everyone will be satisfied with the decision, but I should say that we did much more together than one could have expected several hours ago," said Polish Prime Minister Donald Tusk.
Tusk said there was "no enthusiasm" in Europe for sanctioning Russia, but he called the moves inevitable, given the country's blatant violation of international rights by its actions in Crimea.
British Prime Minister David Cameron agreed, while acknowledging that stiffer sanctions would not only hurt Russia.
"Of course there are consequences for Britain if you look at financial services. Of course there are consequences for France if you look at defense. Of course there are consequences for some European countries if you look at energy," he said.
But he said the EU had to take tough action to counter what he called "the most serious crisis in Europe this century."
"If you are going to stand for something, if you're going to stand up to aggression, you have to look at, you have to consider, all and every one of those areas."
Merkel said if Russia fails to stop its destabilizing measures in Ukraine, "then we will see a far-reaching change in our relationship with Russia, which can also include a broad array of economic measures."
"We don't wish for that to happen," she added.
Russia is Europe's third-largest trading partner and its biggest gas and oil supplier. EU exports to Russia in 2012 totaled 123 billion euros ($170 billion), and European banks have about 200 billion euros in outstanding loans to Russia.
The Obama administration restrictions target a number of unidentified people and entities accused of threatening Ukraine's sovereignty and borders.
And like the raft of EU measures, the U.S. sanctions are flexible: The penalties will increase if Russia continues its aggressive policies but also could be ratcheted down if Moscow pulls back its troops in Crimea and recognizes Ukraine's new government.
The United States had already suspended talks on an investment treaty and threatened further steps. NATO on Wednesday suspended most of its meetings with Russian officials, halting military cooperation and deciding to review all aspects of its relationship with Moscow.
In a symbolic victory in the tussle over Ukraine's future allegiances, the EU leaders agreed to swiftly sign a wide-ranging political association agreement as hoped for by Kiev's new government, cementing closer ties with Europe.
Merkel said the agreement will now be signed "very fast," and Cameron called it a "significant decision." Leaders hope to sign the deal even before the Ukrainian elections planned for May.
Ukraine's ousted President Viktor Yanukovych in November walked away from signing that agreement and chose a bailout loan package from Russia instead, which fuelled the protests that eventually led to the ouster of his government.
A free-trade part of the agreement won't be signed yet, but the EU decided to unilaterally grant Ukraine the financial advantages deriving from it to help boost its ailing economy until the full deal can be signed.
The EU proposed a $15 billion aid package for Ukraine on Wednesday. The U.S. has pledged $1 billion and is working on a more comprehensive package, in coordination with the EU and the International Monetary Fund as Kiev is running out of cash.
Lithuanian President Dalia Grybauskaite underscored fears among Moscow's closest neighbors as she entered the summit.
"Russia today is dangerous," she insisted. "After Ukraine will be Moldova, and after Moldova will be different countries."
Associated Press Writer Angela Charlton contributed reporting.