President Barack Obama on Friday expressed concern about reported Russian military movement inside crisis-torn Ukraine and warned of consequences.

"We are now deeply concerned by reports of military movements taken by the Russian Federation inside of Ukraine," he told reporters at the White House.

"Any violation of Ukraine's sovereignty and territorial integrity would be deeply destabilizing," he said in a brief appearance. "The United States will stand with the international community in affirming that there will be costs for any military intervention in Ukraine."

Regarding what these "costs" may be a senior U.S. official said on Friday that Obama and European leaders would consider skipping a G8 summit in Sochi, Russia planned for this summer if Russia intervenes militarily in Ukraine.

The official, speaking on condition of anonymity, said the United States is consulting with its European partners on potential costs that might be imposed on Russia for any Ukraine intervention.

U.S. officials told Reuters that Washington has seen indications of troop movements from and into Ukraine's Crimea region on Friday, but their numbers are unclear, as are the intentions of those movements.

The senior U.S. official said the U.S. response could also include withholding deeper trade and commerce ties that Moscow is seeking.

Any Russian military movements in Crimea are in keeping with Moscow's existing arrangement with Ukraine on the deployment of military assets in the former Soviet republic, Russia's UN Ambassador Vitaly Churkin said on Friday.

"We are acting within the framework of that agreement," he told reporters after a closed-door meeting of the UN Security Council. He did not give any details or comment on specifics of any Russian military deployments on Ukrainian territory.

Armed men took control of key airports in Crimea on Friday and Russian transport planes flew into the strategic region, Ukrainian officials said, an ominous sign of the Kremlin's iron hand in Ukraine. President Barack Obama warned Moscow there will be costs if it intervenes militarily in Ukraine.

Serhiy Astakhov, a spokesman for the Ukrainian border service, said eight Russian transport planes landed in Crimea Peninsula in southern Ukraine with unknown cargo.

He told The Associated Press that the Il-76 planes arrived unexpectedly and were given permission to land, one after the other, at Gvardeiskoye air base, north of the regional capital, Simferopol. Astakhov said the people in the planes refused to identify themselves and waved off customs officials, saying they didn't require their services.

Russia kept silent on claims of military intervention, even as it maintained its hardline stance on protecting ethnic Russians in Crimea, a territory that has played a symbolic role in its national identity.

Ukraine's UN ambassador said Friday that he told the UN Security Council that Russian military helicopters and transport planes are entering his country and that Russian armed forces seized Crimea's main airport.

Associated Press journalists in Crimea spotted a convoy of nine Russian armored personnel carriers on a road between the port city of Sevastopol, where Russia has a naval base, and the regional capital, Simferopol. The tensions at two Crimea airports apparently caused the closure of airspace over the peninsula.

Russia's Interfax agency cited Serhyi Kunitsyn, a Ukraine presidential envoy to Crimea, telling ATR television that 13 Russian planes carrying 150 Russian troops each landed at Gvardeiskoye air base. That report could not be confirmed.

In Washington, Obama said the United States is deeply concerned by reports of military movements by Russia inside Ukraine. He said any violation of Ukraine's sovereignty and territorial integrity would be destabilizing.

He also said it would violate Russia's commitment to respect Ukraine's borders and would invite global condemnation. Obama said the United States stands with the world community to affirm there will be costs for an intervention.

Russian armored vehicles bearing the nation's tricolor rumbled across Crimea and men described as Russian troops took position at airports and a coast guard base.

U.S. Ambassador to the UN Samantha Power told reporters Friday that the United States wants mediators who will be "seen as independent, credible."

She suggested UN official Robert Serry could be part of the mission. Serry was the Netherlands' first ambassador to Ukraine.

She also said that the Organization for Security and Cooperation in Europe had experts who would be seen as credible.

British UN Ambassador Mark Lyall Grant later said that such a mediation mission would not require the blessing of the UN Security Council. Russia has a council veto and could block action there.

Russian Ambassador Vitaly Churkin said he opposed "imposed mediation."

The sudden arrival of men in military uniform patrolling key strategic facilities prompted Ukraine to accuse Russia of "military invasion and occupation" — a claim that brought an alarming new dimension to the crisis.

Oleksandr Turchynov, who stepped in as president after Viktor Yanukovych fled Kiev last weekend, urged Russian President Vladimir Putin to stop "provocations" in Crimea and pull back military forces from the peninsula. Turchynov said the Ukrainian military would fulfill its duty but would not be drawn into provocations.

Earlier Friday Ukraine's fugitive president resurfaced in Russia to deliver a defiant condemnation of a "bandit coup."