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Oleksandr Turchynov, Ukraine’s interim president, said, “I personally appeal to President [Vladimir] Putin to immediately stop military provocation and to withdraw from the Autonomous Republic of Crimea.” He described the Russian actions as “naked aggression.”

The UN Security Council was preparing to convene an emergency session on the crisis after a request from Ukraine, which warned its territorial integrity was being threatened.

Ukrainian officials told a Crimean television channel 13 planes had landed at a military air base near Simferopol, the regional capital, carrying nearly 2,000 suspected troops.

The Ukrainian foreign ministry said it had officially protested to Moscow over the “violation of Ukraine’s air space” after at least 10 Russian helicopters crossed the border.

Unidentified troops and armour, backed by gunmen in unmarked uniforms, fanned out across Crimea Friday morning, seizing two airports and other key installations in an apparent attempt to assert Moscow’s dominance over the region.

Although no shots were fired and there were no reported injuries, Arsen Avakov, the new Ukrainian interior minister, said the sudden and unannounced Russian deployment amounted to a “military invasion and occupation” of his country’s territory.

U.S. President Barack Obama held a press conference to say he was “deeply concerned” about “reports of military movements taken by the Russian Federation inside Ukraine.” Any intervention would be “destabilizing.”