A top U.S. official says the United States and European leaders may skip the G8 economic summit in Sochi, Russia later this year if the Russian military intervenes in Ukraine.
The official said Friday that deeper trade ties and other commercial projects with Russia could also be canceled.
President Barack Obama says there will be costs for any Russian military moves in Ukraine. In a brief statement to reporters at the White House, he said any violation of Ukrainian sovereignty by Russia would be deeply destabilizing.
He said the United States is deeply concerned by reports of Russian military movements in Crimea and that the Ukrainian people deserve the opportunity to determine their own future.
Ukraine's United Nations ambassador, Yuriy Sergeyev, told an emergency Security Council meeting that Russian attack helicopters and military transport planes have crossed into Crimea.
He described the main airport as being "captured" by Russian forces. Sergeyev said he wants moral and political support from the world community and that what is going on in his country is "awful."
The U.S. ambassador to the U.N., Samantha Power, said the United States is calling for an urgent international mediation mission for Crimea.
Russian ambassador Vitaly Churkin says his country is against what he calls externally imposed mediation.
When asked whether Russia has invaded Ukraine, he said Russia is acting within the framework of a treaty with Ukraine that allows the Russian Black Sea fleet to have a base in Crimea.