WASHINGTON - President Obama said Thursday that new sanctions on Russia will "impose a cost" for its military incursion into Ukraine, and he again called on Russian counterpart Vladimir Putin to de-escalate the crisis.

Speaking at the White House, Obama also said he opposes a referendum that would allow parts of Crimea to break away from Ukraine and join Russia, saying that would violate Ukraine's constitution and international law.

While calling for a "path to de-escalation," Obama also said that "if this violation of international law continues, the resolve of the United States and our allies and the international community will remain firm."

Obama spoke after placing visa restrictions and economic sanctions on Russians who are determined to have been involved in military action in Ukraine.

Describing the Russian incursion into Crimea as "an unusual and extraordinary threat to the national security and foreign policy of the United States," an Obama executive order calls for denying visas and blocking financial transactions of designated Russians.

"The order does not target the country of Ukraine, but rather is aimed at persons — including persons who have asserted governmental authority in the Crimean region without the authorization of the government of Ukraine — who undermine democratic processes and institutions in Ukraine," Obama said in a message to Congress.

The sanctions could also apply to some Ukrainians if they are found to have been involved in efforts to destabilize the country.

The sanctions announced Thursday are limited. They only affect funds in the United States, or controlled by U.S. financial institutions abroad. Visa restrictions and cancellations affect only travel to the United States.

The Obama administration is working with allies on a set of international sanctions on Russia.

Putin himself is, theoretically, a potential target of the new sanctions, though aides said they would be determined on a case-by-case basis. White House spokesman Jay Carney said, "I'm not going to jump ahead and speculate about who might be named."

In his White House statement, Obama again called on Putin to pull back troops to pre-existing Russian bases in Crimea, and to allow international monitors into the region to protect the rights of all ethnic populations.

Putin, who has said he is obligated to protect the rights of Russian-speaking people in Ukraine, has not responded to the administration's offer.

Obama also called on Congress to help apply pressure to Russia by approving aid to Ukraine, and supporting the capacity of the International Monetary Fund to provide Kiev with needed loans.

The U.S. unilateral sanctions come less than a week after the administration called for a $1 billion package of economic and energy assistance to the new government in Kiev.

The U.S. has also suspended trade talks and military cooperation with Russia. It has discussed boycotting a G-8 summit to be hosted by Putin in June in Sochi.

Secretary of State John Kerry discussed the Ukraine unrest Thursday with his Russian counterpart.

The sanctions announcement also comes as a regional parliament in Crimea set a vote on formally cutting ties with Ukraine and joining Russia.

Brendan Buck, a spokesman for House Speaker John Boehner, R-Ohio, said "we welcome this first step, but remain 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."

Boehner has called for increasing U.S. exports of natural gas in order to undercut Russian energy sales to other nations.

Carney said the administration "is pursuing and reviewing a wide range of options in response to Russia's ongoing violation of Ukraine's sovereignty and territorial integrity."

The State Department will put visa restrictions in place against "those responsible for or complicit in threatening the sovereignty and territorial integrity of Ukraine," Carney said. "This new step stands in addition to the policy already implemented to deny visas to those involved in human rights abuses related to political oppression in Ukraine."

Economic sanctions will be aimed at "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," Carney said.

U.S. options include freezing assets and prohibiting Russian firms from doing business in the United States.