"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," Secretary-General Antonio Guterres told reporters.

Russia's mission to the United Nations accused Guterres of violating the UN Charter with his remarks.

"We regret to have heard the statement by the UN Secretary-General," the Russian mission said in a statement. "Regarding the situation around Ukraine the UN Secretary-General has been consistently demonstrating the same selective approach as the countries of the collective West, literally putting himself in the lineup with them."

Russian President Vladimir Putin will sign documents on Friday proclaiming Russia's annexation of four Ukrainian regions, as Moscow rushes to lock in territorial claims that the Ukrainian army is threatening to reverse on the battlefield.

The move, one of the legal steps Russia says will lead to formal annexation of 15% of Ukraine's territory, confirms that Putin is pressing ahead with his war against Ukraine despite suffering a major military reversal this month.

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.

Guterres said the referendums were conducted under Russian occupation and outside Ukraine's legal and constitutional framework.

"Any decision by Russia to go forwards will further jeopardize the prospects for peace," Guterres said.

"It will prolong the dramatic impacts on the global economy, especially in developing countries, and hinder our ability to deliver life-saving aid across Ukraine and beyond."