The Russian government said in a statement that, in reply to U.S. concerns over the possibility of the use of Russian armed forces in Ukraine, Putin "drew his attention to the provocative and criminal actions on the part of ultranationalists who are in fact being supported by the current authorities in Kiev."

According to a statement released Saturday by the White House, Obama "made clear that Russia's continued violation of Ukraine's sovereignty and territorial integrity would negatively impact Russia's standing in the international community."

Lean to the West, or to Russia?

Ukraine, a nation of 45 million people sandwiched between Europe and Russia's southwestern border, has been plunged into chaos since the ouster a week ago of Yanukovych following bloody street protests that left dozens dead and hundreds wounded.

Ukraine has faced a deepening split, with those in the west generally supporting the interim government and its European Union tilt, while many in the east prefer a Ukraine where Russia casts a long shadow.

Nowhere is that feeling more intense than in Crimea, the last big bastion of opposition to the new political leadership. Ukraine suspects Russia of fomenting tension in the autonomous region that might escalate into a bid for separation by its Russian majority.

Ukraine's acting President Oleksandr Turchynov took to the airwaves late Saturday to warn that any Russian military intervention would lead to war.

'The troops are already there'

The crisis set off alarm bells with the world's diplomats, with Ukrainian Ambassador to the U.N. Yuriy Sergeyev calling on member nations of the Security Council to take a stand against what he called Russia's "clear act of aggression.''

"The troops are already there, and their number is increasing every hour," Sergeyev said during an emergency meeting of the Security Council.

Russia now has 15,000 troops Crimea, Yegor Pyvovarov, the spokesman for the Ukrainian mission at the United Nations, told CNN ahead of Saturday's session of the Security Council. He did not say how Ukraine arrived at that number, or whether that included troops already stationed at a Russian base in the region.

Vitaly Churkin, Russia's ambassador to the United Nations, rejected Ukraine's calls to stop Russian intervention. "We can't agree with this at all," he said.

He blamed members of the European Union for causing the bloody street demonstrations in Ukraine.

"It's a difficult situation in the past few hours," Churkin said, claiming that there were Ukrainian forces from Kiev en route to to overthrow the local pro-Russian governments in eastern Ukraine and Crimea and establish new ones that would enforce the power of the new Ukrainian government.

Churkin has said reports of Russian troops taking charge of positions on the ground were rumors and noted that rumors "are always not true."

Crimea's pro-Russian leader asked for help

The Russian parliament vote Saturday came on the day that the newly installed pro-Russian leader of Crimea, Sergey Aksyonov, asked Putin for help in maintaining peace on the Black Sea peninsula, where Russia's fleet is based at Sevastopol.

Security forces "are unable to efficiently control the situation in the republic," he said in comments broadcast on Russian state channel Russia 24. Aksyonov was installed as the region's premier after armed men took over the Crimean parliament building on Thursday.

Aksyonov said that a referendum on greater Crimean autonomy, originally set for May 25, would be moved to March 30.

Yatsenyuk called the Russian presence in Crimea a provocation.

"Ukraine will not be provoked, we will not use force. We demand that the government of the Russian Federation immediately withdraw its troops and return to their home bases," he said during a televised Cabinet meeting.

Airspace in the region reopened Saturday, a day after Ukraine accused Russian Black Sea forces of trying to seize two airports in Crimea but said Ukrainian security forces had prevented them from taking control.

Groups of armed men, dressed in uniforms without identifying insignia, patrolled the airports in Simferopol and the nearby port city of Sevastopol. The men remained at the airports Saturday, but Yevgey Plaksin, director of the airport in Simferopol, said airport services were working.

Obama: Warning to Russia

Senior White House officials say they are looking at a wide range of possible economic and diplomatic measures to present to Obama that would show Putin there is a cost to his actions in Ukraine.