Ukrainians who help Russian-backed referendums to annexe large swathes of the country will face treason charges and at least five years in jail, Ukraine's presidential adviser said, as voting in four regions entered its last day.
"We have lists of names of people who have been involved in some way," presidential adviser Mikhailo Podolyak said in an interview with Swiss newspaper Blick.
"We are talking about hundreds of collaborators. They will be prosecuted for treason. They face prison sentences of at least five years."
Podolyak said Ukrainians who were forced to vote would not be punished. Ukrainians officials have reported ballot boxes being taken door to door and residents being coerced into voting in front of Russian-backed security.
Moscow hopes to annex the provinces of Kherson, Luhansk, Donetsk and Zaporizhzhia, in the east and south, which make up about 15 per cent of Ukraine.
None of the provinces are fully under Moscow's control and fighting has been under way along the entire front line, with Ukrainian forces reporting more advances since they routed Russian troops in a fifth province, Kharkiv, earlier this month.
Russian President Vladimir Putin has issued a veiled threat to use nuclear weapons to protect Russian soil, which would include the four provinces if annexed.
Voting on whether to join Russia began on Friday in the regions and is due to end on Tuesday, with the Russian parliament possibly approving the annexation within days.
The British Ministry of Defence said on Tuesday that Putin is likely to announce the accession of the occupied regions of Ukraine to the Russian Federation during his address to parliament on September 30.
Kyiv and the West have dismissed the referendums as a sham and pledged not to recognise the results.
Ukrainian and Russian forces were locked in heavy fighting in different parts of Ukraine on Tuesday.
Ukrainian President Volodymyr Zelenskiy said the Donetsk region in the east remained his country's -- and Russia's -- top strategic priority, with fighting engulfing several towns as Russian troops try to advance to the south and west.
There were also clashes in Kharkiv region in the northeast -- focus of a Ukrainian counter-offensive this month. And Ukrainian forces pressed on with a campaign to put out of action four bridges and other river crossings to disrupt supply lines to Russian forces in the south.
The Ukrainian Armed Forces southern command said on Tuesday that its counter offensive in Kherson had resulted in enemy losses of 77 servicemen, six tanks, five howitzers, three anti-aircraft installations and 14 armoured vehicles.
Reuters could not immediately verify battlefield reports.