However, Putin remained defiant, telling Obama that not only can Russia send its troops to Crimea, but to all of predominantly Russian-speaking eastern Ukraine.
MOSCOW — Stunned by Russia's swift move into the autonomous province of Crimea and the Russian parliament's endorsement of the action, the United States called on Moscow to "de-escalate tensions by withdrawing its forces" from the region and "refrain from any interference elsewhere in Ukraine."
Speaking by phone with Russian President Vladimir Putin, President Obama expressed concern over "Russia's clear violation of Ukrainian sovereignty and territorial integrity," according to a White House statement.
"President 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," the statement said.
However, Putin remained defiant, telling Obama that not only can Russia send its troops to Crimea, but to all of predominantly Russian-speaking eastern Ukraine due to "the existence of real threats" to Russian citizens in Ukrainian territory, according to a statement on the Kremlin's website.
Earlier Saturday, top U.S. officials gathered at the White House in a high-level meeting to review Russia's military moves into Ukraine and determine a response.
Russia insisted the action was to keep the peace between pro-Moscow Ukrainians and those who want ties to Europe. Sen. John McCain, R-Ariz., disagreed.
"I am deeply concerned that Russia's ongoing military intervention in Crimea may soon expand to eastern Ukraine," he said. "Yesterday, President Obama said that Russia would face 'costs' if it intervened militarily in Ukraine. It is now essential for the president to articulate exactly what those costs will be and to take steps urgently to impose them."
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The United Nations also met Saturday for an emergency session requested by the United Kingdom. In an open meeting later Saturday afternoon, Ukraine's U.N. ambassador, Yuriy Sergeyev, claimed 15,000 Russian troops are now in Crimea.
Russia appeared to have gone ahead with a military takeover of the Crimea Peninsula in eastern Ukraine, where Russian troops from the Black Sea port of Stevasopol were seen moving through various parts of the region.
Andriy Ignatov, a Ukrainian in Crimea, told Kiev Post that the troops are facing no resistance by local residents but aren't being feted either.
"I see no enthusiasm for the arrival of the troops, maybe because they have no insignias," he told the newspaper. "There's no cheering crowds, no flowers. The attitude is pretty much, 'Let's go with the flow.' "
There is concern about what happens next, he added. "People ask, 'Are we going to be part of Russia now? Well, then, OK, as long as there's no fighting.' "
In a late night announcement, Ukraine's interim president Oleksandr Turchynov ordered the nation's armed forces to be prepared for the threat of "potential aggression."
Shortly after speaking in favor of the decision to send Russian troops, Valery Ryazansky, chairman of the Federation Council committee on social welfare, said there's not going to be a war.
"This will be done by those (Ukrainians and Russians) who live there," he said. "We have waited long and patiently hoping that Ukraine could manage on its own. But, unfortunately, it's become clear that without this help and support they cannot manage."
The snap Russian parliament hearing, in which senators did not debate or question the proposal, came shortly after Putin asked parliament for permission to use military force in Ukraine due to the "extraordinary situation" there. Putin said the move is needed to protect ethnic Russians and personnel at a Russian military base in Crimea.
Russian lawmakers also recommended that Putin recall the Russian ambassador to the U.S. in response to Obama's Friday evening speech, where he said the U.S. remained "deeply concerned" about reports that Russian troops had entered Ukraine.
Senior Ukrainian politician Vitali Klitschko called for a "general mobilization" in the country Saturday.
"Parliament must request the army chief to mobilize following Russian aggression against Ukraine," Klitschko said in a statement.
Earlier Saturday, Crimea's pro-Russia leader, Sergei Askenov, declared himself in charge of all security services and made a direct appeal to Moscow.
"I appeal to the president of Russia Vladimir Putin for assistance in guaranteeing peace and calmness on the territory of the autonomous republic of Crimea," Aksenov said in a statement quoted by Russian news agencies.
In Kiev, the interim government that replaced exiled president Viktor Yanukovych said Russia is violating its sovereignty.
"We demand that the Russian government and authorities recall their troops and return them to their bases," interim Prime Minister Arseniy Yatseniuk said Saturday in remarks published by Interfax, Ukraine's official agency.
"There is inappropriate presence of Russian military servicemen in Crimea now. This is nothing but a violation of the basic provisions of the agreement on the Russian Black Sea Fleet's temporary presence on the Ukrainian territory," Yatseniuk said.
Ukraine closed its airspace over Crimea on Saturday after border officials said at least eight Russian transport planes landed Friday at a military base, delivering unknown goods and personnel. Ukraine's Crimea province has an ethnic Russian majority and hosts the Russian navy's Black Sea fleet.
All flights to and from the provincial capital of Simferopol were canceled a day after unidentified gunmen — suspected of being Russian forces — took over the region's main airport and a military airport outside the naval city of Sevastopol.
Regardless, the situation was tense in the region as Aksionov reassured local residents that he had the situation under control. Most residents of the Black Sea Peninsula have linguistic and cultural ties to Russia with a clear majority self-identifying as ethnic Russian.
"The authorities are at their desks working," he said at a meeting televised by state broadcaster Krym TV. "We guarantee the people, irrespective of their ethnic background, a sober, realistic and just approach in resolving their issues. I am especially addressing the Crimean Tatars: I assure you that nothing threatens you."
Meanwhile, protests broke out in the eastern Ukrainian city of Donetsk on Saturday after thousands of people gathered to press for a referendum on seceding from Ukraine, carrying Russian flags and banners expressing support of pro-Russian protesters in Crimea. Residents are also pushing for new local elections.
In the stronghold of ousted president Yanukovych's Party of Regions in the east's regional capital of Kharkiv, clashes broke out between pro-Russian protesters and pro-Maidan demonstrators. A Ukrainian flag on a government building was torn down and replaced with a Russian one, local media reported.