In a televised statement on Friday, US President Barack Obama said that Washington was "deeply concerned by reports of military movements taken by Russian forces" in Crimea.

Any violation of Ukraine's sovereignty "would be deeply destabilizing, which is not in the interest of Ukraine, Russia or Europe," the president continued.

"The United States will stand with the international community in affirming that there will be costs for any military intervention in Ukraine," Obama told reporters.

A senior US official reportedly told the news agency AFP that President Obama and key European leaders may skip the Group of Eight (G8) summit in Sochi if Russia infringes on Ukraine's sovereignty. The official went on to say that Washington may also withhold deeper economic ties with Moscow.

Ukraine's ambassador to the UN, Yuriy Sergeyev, accused Moscow of deploying military transport planes and attack helicopters to the Crimean Peninsula earlier on Friday.

The two major airports in the Crimea were no longer under the control of Ukraine's interim government, according to Sergeyev. He told reporters outside of the Security Council that the main airport in Crimea had been "captured by Russian armed forces."

Russia denies 'aggression'

Meanwhile, Russia's ambassador to the UN, Vitaly Churkin, dismissed Sergeyev's allegations of aggression on the part Moscow.

"We have an agreement with Ukraine on the presence of the Russian Black Sea fleet in Sevastopol and we are acting within the framework of that agreement," Churkin said. In response to a question, he added that he did not have specifics on whether or not additional Russian forces had landed in Crimea.

Churkin accused Ukraine's interim government of ignoring a political agreement forged by the EU and Russia, which ended days of violence in Kyiv. The agreement had called for a national unity government, early presidential elections, and for "radicals" to be disarmed, according to the Russian ambassador.

"Unfortunately all of those things were discarded and it created the current situation of instability," Churkin said.

He went on to say that Ukraine's interim government only contained representatives from central and western Ukraine, which had raised concern among Russian-speaking communities in the country's east. Churkin called for the February EU-sponsored political agreement to be implemented.

slk/lw (AP, dpa)