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Human rights commissioner Lyudmila Denisova said Russian forces were distributing propaganda material telling people in occupied Kherson that Moscow was acting for their benefit.
She said leaflets handed out “blame the Ukrainians themselves and their chosen government” for the aggression against their country, echoing the Kremlin’s messaging about the invasion.
Russia in 2014 claimed control of the Crimean peninsula from Ukraine after a referendum supposedly showed public backing for an annexation.
The poll was widely rejected as a sham by western countries, who said it was a breach of Ukraine’s constitution and that voters hardly had a free choice when Russian troops were on their soil.
Ms Denisova said there was evidence that Russia was planning a similar vote in the Kherson region in early May, citing witnesses who said ballot papers were already being prepared at a local printing house.
Ukraine’s Defence Ministry meanwhile said Russia was planning to falsify results by collecting personal data on residents under cover of distributing humanitarian aid.
As well as spreading propaganda to locals, Russia says it has donated humanitarian aid to people in Kherson including food, blankets and school supplies for children.
Residents have expressed concern that they will be mobilised into Russia’s armed forces if it succeeds in orchestrating a pro-Kremlin result, the Ukrainian ministry said.
Ukrainian authorities say any such ballot would be illegal, because the constitution requires the whole country to vote on any territorial changes and three million to people submit a petition to trigger such a poll.
Ukraine’s allies have raised similar concerns. The US delegate to the Organisation for Security and Co-operation in Europe said there were reports of “sham elections” being planned in Kherson in an echo of the Crimea vote.
British military intelligence suggested last month that Russia was planning to give Kherson a similar status to Crimea and the breakaway territories of Donetsk and Luhansk in eastern Ukraine.
Russian forces seized control of Kherson in early March in one of their most significant military victories of the war, although some protests have taken place against their occupation.
A power grab in Kherson could involve Russian forces preventing entry and exit from the city and disconnecting communications, Ms Denisova said. Western countries said there was no independent monitoring of the 2014 Crimea vote.
If Russia stage-manages the result of a “pseudo-referendum”, it could establish a so-called People’s Republic of Kherson, she said – mimicking the names used by Moscow for Donetsk and Luhansk.
Russia recognised those two regions as independent entities days before it invaded and cited fighting with Ukrainian forces there as one of the justifications for its offensive.
The Kremlin claims it is liberating Ukrainian cities from nationalists, Nazis and foreign mercenaries and neutralising threats to Russia. Ukraine and its allies reject these as spurious excuses for a war of aggression.