Ukraine's acting defence minister says Russia is continuing its build-up of troops in Crimea.

Ihor Tenyukh says they now number up to 22-thousand servicemen - almost double the number allowed under an agreement between Russia and Ukraine.

News of the build-up comes as voters on the peninsula vote in a referendum on whether to re-join Russia. 

It's expected the majority of voters in the largely Russian-speaking region will accept the proposal.

The poll has been condemned as illegal by western leaders but Moscow says it complies with international law.

Crimea Expert Johannes Andersen says the referendum is part of a broader Russian policy.