President Vladimir Putin will sign documents on Friday proclaiming Russia's annexation of four Ukrainian regions.
The move, one of the legal steps Russia says will lead to formal annexation of 15% of Ukraine's territory, confirms that Putin is doubling down on his war against Ukraine.
The annexation, after what Kyiv and Western countries say were phony referendums staged at gunpoint on Russian-held Ukrainian territory, has been rejected in the West as an illegal seizure of land captured in war.
Ukrainian President Volodymr Zelenskiy summoned his security and defense chiefs for an emergency meeting to be held on Friday, and promised a robust response to a step he says has killed off chances of reviving peace talks.
The votes "are worthless and do not change reality. The territorial integrity of Ukraine will be restored. And our reaction to recognition of the results by Russia will be very harsh," Zelenskiy said in a statement.
The United Nations Secretary-General warned that if Russia moves ahead with its plans to annex the four Ukrainian regions, it would mark a "dangerous escalation" that would jeopardize the prospects for peace in the region
"Any decision to proceed with the annexation of Donetsk, Luhansk, Kherson and Zaporizhzhia regions of Ukraine would have no legal value and deserves to be condemned," Antonio Guterres told reporters.