WASHINGTON - The proposed upcoming referendum in Crimea would violate Ukraine's Constitution and the international law, and any discussion on the future of Crimea should include the Ukrainian government, US President Barack Obama said Thursday.

Speaking on the Ukraine crisis at the White House, Obama said the US has been mobilizing the international community to " condemn" Russia's "violation of international law" and support the people and government of Ukraine since Russia's "intervention" of Crimea.

Crimea decided Thursday to hold a referendum on March 16 on whether to join Russia or remain part of Ukraine as the parliament of Crimea, a Ukrainian autonomous republic, voted to join Russia.

The US President noted that, in order to impose costs on Russia for its actions, he signed an executive order earlier Thursday to authorize sanctions on "the individuals and entities responsible for violating the sovereignty and territorial integrity of Ukraine or for stealing the assets of the Ukrainian people."

Under the order, the US is putting in place visa restrictions and moving to freeze US assets of those involved in Russia's move on Ukraine, the White House said earlier Thursday.

"These decisions continue our efforts to impose a cost on Russia and those responsible for the situation in Crimea," Obama said, adding that the US took the steps in coordination with its European allies.

Obama also called for allowing international monitors into all of Ukraine, including Crimea, to ensure that the rights of all Ukrainians are respected, including those of the Ukrainian Russians.

He urged the governments of Ukraine and Russia to begin consultations on resolving the crisis with the participation of the international community, adding Russia would maintain its basing rights in Crimea if it abides by its agreements with Ukraine and respect the Ukrainian sovereignty and territorial integrity.

"And the world should support the people of Ukraine as they move to elections in May," the US president said.

Obama said if the Russian "violation" of international law continues, the resolve of the US and its allies would remain firm as Washington has taken steps to reaffirm its commitment to security and democracy of its eastern European allies and to support Ukraine.

Russia, in the meantime, accused the United States of applying double standards to Moscow's assertions about the developments in Ukraine.

"The (US) State Department is trying to play on a shamelessly one-sided interpretation of events as though there wasn't abundant evidence of outrages perpetrated by radical nationalists (in Ukraine)," Russian Foreign Ministry spokesman Alexander Lukashevich said Thursday in a statement, referring to a statement released by US government criticizing Russian President Vladimir Putin.

The United States neither had nor could have the right to moralize about compliance with international law and respect for the sovereignty of other countries, said the spokesman.

The mostly Russian-speaking Crimea, home to Russia's Black Sea Fleet, has become the epicenter of the ongoing crisis in Ukraine since President Viktor Yanukovych was ousted by the parliament on Feb. 22.