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 on Thursday 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.

Speaking in the White House Briefing Room, Obama said his administration and European nations concerned about Russia’s intervention in Ukraine were “moving forward together” to sanction Russia, namely officials behind the military move into Crimea. Earlier in the day, Obama signed an executive order authorizing such sanctions, which he hopes will encourage Putin to pull back Russia-aligned forces in Crimea and allow international observers to insure the protection of Ukrainian, including ethnic Russians.

Obama called such steps, if supported by Putin, the most effective to end the crisis, which European officials have called the most serious since the Balkan wars of the 1990s.

“If this violation of inter nation law continues, the resolve of the United States and its international allies will remain firm,” Obama said.

The president also called on Congress to expand the lending capacity of the International Monetary Fund to provide loan guarantees and other aid to Ukraine.