•Ukraine's Western allies have denounced the move as a land grab and refused to recognise the results of the ballots.
•US Secretary of State Antony Blinken said it was a "dangerous escalation".
Russian-occupied areas of Ukraine are staging so-called "referendums" on whether to join Russia, as Moscow pushes ahead with an apparent bid to annex territory from its neighbour.
Voting begins on Friday in four regions of Ukraine in which Russia has gained ground during its invasion.
Ukraine's Western allies have denounced the move as a land grab and refused to recognise the results of the ballots.
US Secretary of State Antony Blinken said it was a "dangerous escalation".
Mr Blinken told the UN Security Council on Thursday that no Russian claim to Ukrainian soil could take away Ukraine's right to defend itself.
In his Thursday night address, Ukrainian President Volodymyr Zelensky said the ballots were a "sham" and undemocratic.
The snap votes were called earlier this week in the eastern regions of Luhansk and Donetsk - as well as Zaporizhzhia and Kherson in the south.
The four regions represent around 15% of Ukraine, or an area the size of Hungary, according the Reuters news agency.
Voting is scheduled to take place over five days.
Russian media reported that electoral officials would go from door-to-door with portable ballot boxes from Friday to Monday.
Polling stations will only operate on the fifth day, 27 September, with officials citing security reasons.
Hundreds of stations are scheduled to open that day, with voters also able to cast ballots in regions outside their own - and refugees eligible to vote in parts of Russia itself.
Parts of Luhansk and Donetsk - where voting papers will be written in Russian only - have been under the control of Russian-backed separatists since 2014, with more territory taken this year.
Moscow took parts of Zaporizhzhia and Kherson regions when it invaded in February.
If Russia proceeds to claim Ukrainian land as its own, it could make the claim that its territory - not just its army - is coming under attack from Western weapons supplied to Ukraine.
It is feared this could cause a further escalation in the war.
Russia annexed Crimea in southern Ukraine in 2014, through another so-called referendum that was similarly condemned as illegitimate by the international community.
The fresh ballots come after Wednesday's announcement from Russian President Vladimir Putin of a partial military mobilisation, which could see 300,000 people called up to serve in the war.
The move prompted a rush to the borders by men hoping to avoid service, as well as rare unrest on the streets of Russia that was followed by reports of hundreds of arrests.