Crimea is set to vote on whether to re-join Russia or stay with Ukraine – a referendum condemned as “illegal” by Kiev and the West but backed by Moscow.
Russian troops have taken de facto control of the majority ethnic-Russian region, and voters are expected to support leaving Ukraine.
Crimean Tatars are boycotting the vote, pledging their allegiance to Kiev.
Russia earlier vetoed a draft UN resolution criticising the vote – the only Security Council member to do so.
The US-drafted document was supported by 13 Council members. China, regarded as a Russian ally on the issue, abstained from the vote.
The US and EU have warned they would slap further tough sanctions against Russian officials if the referendum goes ahead.
Russia intervened in the Crimean peninsula by seizing control of government buildings and blocking Ukraine’s troops at their bases after the fall of Ukraine’s pro-Moscow President Viktor Yanukovych on 22 February.
However, the Kremlin officially denies deploying extra troops there, describing them as Crimea’s “self-defence forces.”