And he declared illegal the plan from Putin's allies in Crimea for a March 16 referendum on seceding from Ukraine and joining Russia.
In Europe, 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 sanctions by the Obama administration, which imposed visa restrictions on pro-Russian opponents of the new Ukrainian government in Kiev and cleared the way for financial sanctions.
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.