Amid reports of Russian military movement in Ukraine's Crimea region, President Obama today warned Moscow there "will be costs" if Russia intervenes in Ukraine's affairs.

"We are now deeply concerned by reports of military movements taken by the Russian Federation inside of Ukraine," Obama told reporters in the White House Briefing Room. "Any violation of Ukraine's sovereignty and territorial integrity would be deeply destabilizing, which is not in the interests of Ukraine, Russia or Europe. It would represent a profound interference in matters that must be determined by the Ukrainian people.

"It would be a clear violation of Russia's commitment to respect the independence and sovereignty and borders of Ukraine, and of international laws," he added. "The United States will stand with the international community in affirming that there will be costs for any military intervention in Ukraine."

Making his first public comments on the crisis since Ukrainian President Viktor Yanukovich was pushed from power, Obama described the situation as "fluid."

Russian military aircraft landed today at the military air base near Simferopol, Ukraine, U.S. officials told ABC News. They would not make an assessment of the flights or specify how many aircraft landed, but said the number was likely lower than the 13 reported by other media outlets.

(Ivan Sekretarev/AP Photo)

"We're monitoring the situation in the Crimea," a Pentagon official said. "We're not prepared to offer an assessment or characterize this fluid situation."

Vice President Biden spoke with the prime minister of Ukraine today to "assure him that in this difficult moment, the United States supports his government's efforts and stands for the sovereignty, territorial integrity and democratic future of Ukraine," Obama said.

The Obama administration has cautioned Russia not to cross a "line" in Ukraine, but declined to speculate how the U.S. would respond if Russia ignores the warning.

"I'm not going to speculate about what happens if this happens," press secretary Jay Carney told reporters today. "We've made clear that the United States supports the territorial integrity of Ukraine and the sovereignty of Ukraine. And we have made clear to Russia that we believe it would be a grave mistake to intervene in any way."