During a televised statement, Obama also said there would be a cost to Russia if it continues its military incursion into the autonomous, pro-Moscow Crimea region of Ukraine.
"The proposed referendum on the future of Crimea would violate the Ukrainian constitution and violate international law," Obama said. "Any discussion about the future of Ukraine must include the legitimate government of Ukraine.
"In 2014 we are well beyond the days when borders can be redrawn over the heads of democratic leaders," he said.
Earlier Thursday, the United States took steps to impose sanctions against individuals and entities and visa restrictions against those responsible or complicit in escalating tensions in Ukraine after the overthrow of President Viktor Yanukovych, who fled to Russia. Throughout the week, thousands of Russian troops have been deployed to Crimea. Russian President Vladimir Putin has denied there is a Russian military presence in the Crimea peninsula, but the soldiers – who aren't wearing any identifiable insignias – are driving in Russian military vehicles with Russian license plates.
Obama said the decisions on sanctions and visa restrictions "continue our efforts to impose a cost on Russia and those responsible for the situation in Crimea. And they also give us the flexibility to adjust our response going forward based on Russia's actions."
"I want to be clear that there's also a way to resolve this crisis that respects the interests of the Russian Federation, as well as the Ukrainian people," Obama said. "Let international monitors into all of Ukraine, including Crimea, to ensure the rights of all Ukrainians are being respected, including ethnic Russians. Begin consultations between the government of Russia and Ukraine, with the participation of the international community. Russia would maintain its basing rights in Crimea, provided that it abides by its agreements and respects Ukraine's sovereignty and territorial integrity. And the world should support the people of Ukraine as they move to elections in May."
Obama also urged Congress to support the International Monetary Fund's capacity to lend resources to Ukraine and to provide aid for the Ukrainian government.
"Today, the world can see that the United States is united with our allies and partners in upholding international law and pursuing a just outcome that advances global security and the future that the Ukrainian people deserve," Obama said. "That's what we're going to continue to do in the days to come until we have seen a resolution to this crisis."
Already the United States has suspended trade talks and U.S.-Russia military-to-military activities and put on hold participation in preparing for the Group of Eight summit in Sochi, Russia, in June.