"What we are going to do is mobilise all of our diplomatic resources to make sure that we've got a strong international coalition that sends a clear message."
In response to the Ukraine crisis, the Obama administration has focused on providing economic aid to Ukraine and imposing sanctions against Russia.
On Monday, Obama announced sanctions against 11 Russian and Ukraine officials who the US said were responsible for threatening Ukraine's sovereignty and territorial integrity. He indicated that further sanctions could be on the way.
In defiance of condemnations and further sanctions by the West, Russian President Vladimir Putin signed a treaty on Tuesday with leaders of Crimea to accept the Republic of Crimea and the city of Sevastopol as part of the Russian territory.
The accession treaty followed Monday's decision by the Crimean parliament to declare independence from Ukraine and Sunday's referendum in the peninsula of Crimea showed an overwhelming 96.77% of Crimeans in favor of rejoining Russia.
Crimea, home to the Russian Black Sea Fleet and historically part of the Russian Federation, was transferred to Ukraine in May 1954, then a republic of the Soviet Union.
The White House said on Wednesday that Russia was "directly responsible" for casualties in Crimea amid reports that two people had been killed by sniper shots in Simferopol.