Speaking from the White House on Thursday, President Obama called “unconstitutional” a planned referendum in the Crimea over whether people there want to remain part of Ukraine, and reiterated a series of steps Russian President Vladimir Putin could take to “de-escalate” the conflict in the region. (The Associated Press)

President Obama authorized the Treasury Department on Thursday to impose sanctions on “individuals and entities” responsible for Russia’s military takeover in Crimea or for “stealing the assets of the Ukrainian people.”

The financial measures, and a separate ban on U.S. visas, are part of the administration’s effort to squeeze Russia into pulling back its troops in Crimea, an autonomous, pro-Russia region of Ukraine that does not recognize the country’s new Western-backed leaders.

The U.S action came as European Union leaders gathered in Brussels to consider similar measures. “Our international unity is on display at this important moment,” Obama said in a statement at the White House.

But E.U. measures ultimately fell somewhat short of those imposed by the United States. In a communique, the Europeans said that they will “decide on additional measures, such as travel bans, asset freezes” and cancellation of an upcoming E.U.-Russia summit, “in the absence” of Russian movement and results within the next few days.

Obama dismissed as “unconstitutional” a planned referendum in Crimea over whether people there want to remain part of Ukraine, saying that “any discussion about the future of Ukraine must include the legitimate government of Ukraine.”

“In 2014,” he said, “we are well beyond the days when borders can be redrawn over the heads of democratic leaders.”

After his public comments, Obama spoke for an hour with Russian President Vladimir Putin, the second lengthy conversation between the two leaders this week. The White House said he told Putin that U.S. steps were taken in response to Russia’s “violation of Ukraine’s sovereignty and territorial integrity.” Obama, the White House said, again outlined what U.S. officials have called a “way out” for Putin, including direct talks with the Ukrainian government, the return of Russian forces to their bases, and allowing international monitors to “ensure that the rights of all Ukrainians are protected, including ethnic Russians.”

Secretary of State John F. Kerry met for the second time in two days with his Russian counterpart, Sergei Lavrov. After the meeting, held in Rome, Lavrov said that he considered the sanctions a “threat,” but that Kerry had assured him that lists of targets do not exist.

“We have made suggestions to Foreign Minister Lavrov,” Kerry said, and “we agreed to stay in close touch in order to see whether there is a way forward to try to get to the negotiating table.” The crisis in Ukraine largely overshadowed a gathering in Rome initially called to discuss strengthening the government in Libya.

Senior administration officials acknowledged that they have not determined who would be subject to the asset freezes outlined in Obama’s order. Instead, an official said the prospect of sanctions “should be leading people in Russia and people in Crimea to be asking whether or not they’re going to see their names in the designation.”

The administration will impose costs for what the Russians “have already done in Crimea,” an official said, but the main purpose of the measure appeared to be to pressure Russia to open talks with the Ukrainian government and refrain from expanding its troop presence into Russian-speaking areas of eastern Ukraine.

“It also gives us flexibility” to expand the measures in the event of further Russian actions, the senior official said.

Administration officials previously said sanctions would not apply to Putin or other top Russian officials. “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. Officials spoke on the condition of anonymity to focus attention on public statements by Obama and Kerry.

The authorized sanctions allow the Treasury Department to freeze all U.S. or U.S.-controlled assets of any person or entity who led or assisted efforts to undermine the security or territorial integrity of Ukraine, or asserted control over any part of Ukraine without the authorization of the government in Kiev.

The sanctions are the toughest measure yet as the United States and its European allies took steps to show their disapproval of Russia’s military moves in Crimea.

Europeans have been divided between those major countries with significant economic equities in Russia and Eastern European governments that fear a Soviet-style expansion by Russia.

“The discussions were stormy,” Polish Prime Minister Donald Tusk said after the E.U. meeting. “Maybe not everybody will be satisfied, but we achieved more than could have been expected just a few hours ago.”

British Prime Minister David Cameron said that if talks between Russia and Ukraine didn’t progress quickly, then “asset freezes, travel bans, those things mentioned by the Americans, will be very firmly on the agenda.”

Germany has been singled out in some news accounts as reluctant to move toward significant sanctions. But German officials insisted Thursday that unity between the United States and Europe was their primary objective and said that accounts of Germany’s economic interests and energy dependence on Russia have been overstated.

European resolve was apparently stiffened by the Crimea referendum announcement, which German Chancellor Angela Merkel called “illegal and incompatible to Ukraine’s constitution,” the Guardian reported from Brussels.

Also on Thursday, the government of Lithuania announced the arrival of six U.S. F-15 fighter jets and two military refueling aircraft, reinforcement for an existing four-plane air patrol that NATO operates over the Baltic states to ensure their security.

The Pentagon has said it will step up participation in upcoming military exercises with Poland, although spokesman Rear Adm. John F. Kirby denied Polish news media reports that as many as a dozen U.S. F-16s were on their way.

The White House is pressing Congress to support a $1 billion aid package for Ukraine’s interim government. The House on Thursday overwhelmingly approved a financial aid package for Ukraine, authorizing up to $1 billion in loan guarantees. The 385- to 23 vote was the first congressional action on Ukraine, and the bill will now go to the Senate, where some members may propose a broader package of relief.

NATO Secretary General Anders Fogh Rasmussen on Thursday called the crisis in Ukraine “the gravest threat to European security since the end of the Cold War.”

After a meeting in Brussels with Ukrainian Prime Minister Arseniy Yatsenyuk, Rasmussen said that “we will strengthen our efforts to build the capacity of the Ukrainian military, including with more joint training and exercises.”

Yatsenyuk told reporters that “it’s clear that no military option is on the table, but it’s clear that it’s up to the Russian government to make the first step back. . . . We urge the Russian government to pull back its forces, back into barracks.”

The Russians “started this,” Yatsenyuk said. “They need to put an end to this.”

Asked whether Ukraine intends to apply for NATO membership, he said that “it’s not on our radars.”

Gearan reported from Rome. David Nakamura and Ed O’Keefe in Washington and Karla Adam in London contributed to this report.