"Any violation of Ukraine's sovereignty and territorial integrity would be deeply [de]stabilizing," Obama said in a hastily arranged statement delivered from the White House. Such action by Russia would not serve the interests of the Ukrainian people, Russia or Europe, Obama said, and would represent a "profound interference" in matters he said must be decided by the Ukrainian people.
"Just days after the world came to Russia for the Olympic Games, that would invite the condemnation of nations around the world," Obama said. "The United States will stand with the international community in affirming that there will be costs for any military intervention in Ukraine."
Despite Obama's hard line, America's options in Crimea are limited. As my colleague Josh Keating noted, the United States is very unlikely to intervene, even though Russia's actions probably violate a U.S.-Russian agreement to respect Ukraine's sovereignty within its current borders. With no actual weapons at his disposal, then, Obama has continued to deploy verbal warnings—a fairly minor tool in the face of an international crisis.
For more information on the crisis, see Josh Keating's Ukraine/Crimea Explainer.