Volodymyr Zelenskyy says any "pseudo referendums" held in Russian-occupied parts of Ukraine would be a "slap in the face for the international community".
With Russia controlling large swathes of the eastern Donbas region and parts of southern Ukraine, officials have raised the possibility of holding referendums.
The referendums would be votes on whether locals would want to join Russia and it was a move used when Russia annexed Crimea back in 2014.
But in his nightly address, Zelenskyy, fresh from meeting UN chief Antonio Guterres and Turkey president Recep Tayyip Erdogan, hit out at the move.
"I called on both Mr President Recep Tayyip Erdogan and Mr Secretary General of the United Nations Antonio Guterres to voice the strictest possible position regarding Russia's planned pseudo-referendums in the occupied territory," he said.
"Any pseudo-referendum will be a slap in the face of the international community," he added.
Russian forces hold most of the Kherson region in southern Ukraine and officials in charge have suggested a referendum on joining Russia could be held within the coming weeks or months.
In Donbas, Russian proxies seized chunks of territory in 2014, held independence referendums and proclaimed "people's republics" in Luhansk and Donetsk regions.
The Kremlin recognised the republics on the eve of the February invasion.
The governor of the Luhansk region — almost entirely under Russian control for several weeks — suggested earlier in August that Russia was preparing for a new referendum in newly captured areas and was offering residents benefits for taking part.
Turkish President Tayyip Erdogan said he will discuss the Zaporizhzhia nuclear plant issue with Russian President Vladimir Putin, after meeting with
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