The White House on Monday announced that President Obama had signed an executive order imposing economic sanctions and travel restrictions against eleven Russian government officials.

“Today’s actions send a strong message to the Russian government that there are consequences for their actions that violate the sovereignty and territorial integrity of Ukraine, including their actions supporting the illegal referendum for Crimean separation,” the White House said in a statement.

The move followed Sunday’s referendum vote in Crimea, where a vast majority of the peninsula’s ethnically Russian population voted to break away from Ukraine and join Russia.

The United States and its Western allies have blasted the vote as illegitimate.

In a phone call on Sunday, President Obama warned Russian President Vladimir Putin that the “additional costs” would be forthcoming. Obama said the U.S. and its allies would “never” recognize the referendum vote, which took place while Russian troops were in place, and argued it was a violation of international law and Ukraine’s constitution.

“President Obama has been crystal clear that we will impose costs on those who undermine the sovereignty and territorial integrity of Ukraine, and today’s actions follow through on that commitment,” said Under Secretary for Terrorism and Financial Intelligence David S. Cohen. “The steps taken by the separatist leaders designated today and former President Yanukovych violate Ukraine’s sovereignty and territorial integrity. While we continue to urge a de-escalation of the situation in Ukraine, the conduct of those sanctioned today cannot go unanswered.”