Two million Crimeans started casting ballots Sunday morning local time, to vote in a referendum on whether to stay as an independent state in Ukraine, or to join Russia.
Backed by Moscow and opposed by Ukraine's new government and its western allies, the referendum is widely expected to pass as a majority of those casting ballots are believed to be pro-Russian.
The Russian government on Saturday vetoed a UN resolution criticizing the referendum, and U.S. Secretary of State John Kerry said Friday that Russia would face sanctions if the vote went ahead.
He also warned of an even greater response, if Russia sends troops into Ukraine.
While diplomats were sparring verbally over the situation in Crimea, Russian troops were taking over a gas installation in a strip of land between the peninsula and Russia's mainland.
The Kherson region is a major supplier of natural gas and water to Crimea.
The heavy presence of armed pro-Russian forces has ratcheted up tension on the Crimean peninsula.
Two were killed and several others injured, including a policeman, in a shooting on Friday night.
Local news reports say the gunshots rang out after a skirmish between pro-Russia and pro-Ukraine demonstrators.
Shin Se-min, Arirang News.