President Barack Obama has warned Russia against any military intervention in Ukraine.
The US president said he was deeply concerned by reports of Russian military movements inside Ukraine.
Ukraine’s acting President Oleksandry Turchynov has accused Russia of deploying troops to the Ukrainian region of Crimea and trying to provoke Kiev into “armed conflict”.
Crimea’s pro-Moscow PM Sergey Aksyonov has asked Russian authorities for help in maintaining peace in the region.
“I appeal to the president of the Russian Federation, Vladimir Putin, to provide assistance in ensuring peace and tranquility on the territory of the Autonomous Republic of Crimea,” Sergey Aksyonov said in a statement.
Sergey Aksyonov, who was appointed by the Crimean parliament on Thursday, also said he was taking control of Crimea’s interior ministry, armed forces, fleet and border guards “on a temporary basis”.
The new cabinet in Ukraine is due to meet for the first time on Saturday to discuss the deepening crisis over Russia’s reported military deployments.
Russia’s UN ambassador earlier insisted any troop movements in Crimea were within an existing arrangement with Ukraine.
Overnight reports said armed men in unidentified military uniforms had seized another airfield.
On Friday they took over airports in the Crimean capital, Simferopol, and Sevastopol, where Russia’s Black Sea Fleet is based.
Ukrainian media citing local officials said 13 Russian aircraft carrying nearly 2,000 suspected troops had landed at a military air base near Simferopol. The report remains unconfirmed.
Russian armored vehicles and helicopters were also seen in and around Simferopol and Sevastopol.
Flights from and to Simferopol were cancelled with airlines saying airspace over the peninsula had been closed.
The armed men also moved in on Crimea’s parliament, state television building and telecommunication centers.
Speaking from the White House, Barack Obama commended Ukraine’s interim government for its “restraint”.
“Any violation of Ukraine’s sovereignty and territorial integrity would be deeply destabilizing, which is not in the interests of Ukraine, Russia or Europe,” he said.
“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.”
Barack Obama added: “Just days after the world came to Russia for the Olympic games, it would invite the condemnation of nations around the world. And, indeed, the United States will stand with the international community in affirming that there will be costs for any military intervention in Ukraine.”
The president did not spell out what any US response might be.
In a TV address on Friday, Ukraine’s interim President Oleksandr Turchynov said Moscow wanted the new government to react to provocations so it could annex Crimea.
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