If Russia moves ahead with its plans to annex four Ukrainian regions, it would mark a "dangerous escalation" that would jeopardize the prospects for peace in the region, the United Nations Secretary-General said on Thursday.

"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.

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.

"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."