Ukraine has put its troops on high combat alert, appealed to Nato for help and warned of war after Russian President Vladimir Putin received parliamentary approval to invade Ukraine.
Russia has effectively seized control of Crimea, which was part of Russia until 1954. Russian soldiers - although not identified as such - have surrounded Ukraine military bases in the province and taken over three major airports.
President Obama and other western leaders have spoken with Putin and warned him against further military intervention, with Obama making economic threats. U.S. legislators have condemned Russia's actions and have called for sanctions, but have stopped short of demanding military action.
The developments in Ukraine have troubled but not roiled global markets so far, although the country's main stock index and currency have been hit badly. Gas and oil are a particular focus given that Ukraine is a key east-west energy route.