The Ukraine has put its military on full combat alert and warned Russia that if it invades, it means war.
President Obama called the Kremlin on the hot-line to condemn their actions and to tell President Putin to backdown. (Daily Mail)
I'm not sure what the US hopes to do. Russian forces have already taken over two airports and have massed 150,000 soldiers on the border. Even if the White House had the desire to intervene, we don't have enough forces in the region to effectively counter the Russians. Military actions by US forces would lead to casualties both on the Ukraine and Russian sides, hardly something the US has had trouble dealing with in both Iraq and Afghanistan.
Russia sees it as their right of protection so I doubt this will be solved by any diplomatic measures from the US (which Russia sees as encouraging the separatist factions in the Ukraine). Putin has a vision of putting Russia back on the map as a world power and the Ukraine is his vehicle to do it. Obama wants to pull the US out of being the world's policeman and Putin knows it.
The unknown in this is should the US try to intervene militarily, would China then attack Japan or North Korea head south?