A referendum condemned as "illegal" by Kiev and the West but backed by Moscow is being held on Sunday, reports BBC.
Russian troops have had taken control of the majority ethnic-Russian region. Voters are expected to support leaving Ukraine.
Crimean Tatars are boycotting the vote, pledging their allegiance to Kiev.
The only Security Council member to do so was Russia who earlier vetoed a draft UN resolution criticising the vote.
The US-drafted document was supported by 13 Council members. China was regarded as a Russian ally on the issue, abstained from the vote.
The US and EU have warned they would put forward further tough sanctions against Russian officials if the referendum goes ahead.
Russia intervened in the Crimean peninsula by seizing control of government buildings and blocked Ukraine's troops at their bases after the fall of Ukraine's pro-Moscow President Viktor Yanukovych on February 22.
However, the Kremlin officially denies deploying extra troops there, describing them as Crimea's "self-defence forces".
Kiev talked against the Russian forces of seizing a village just north of Crimea, describing the move as "the military invasion".
Tens of thousands of opponents and supporters of Russia's actions in Ukraine held rival rallies in Moscow. Pro-Ukrainian activists warned of provocations during a pro-Russian rally in Donetsk, eastern Ukraine, on Sunday.
Voters were being asked whether they would like Crimea to rejoin Russia. A second question asked whether Ukraine should return to its status under the 1992 constitution, which would give the region much greater autonomy.
