Russia’s plan to annex four Ukrainian regions would mark a “dangerous escalation” in the war in Ukraine that would jeopardise the prospects for peace, the United Nations secretary-general has warned.

“Any decision to proceed with the annexation of Donetsk, Luhansk, Kherson and Zaporizhia regions of Ukraine would have no legal value and deserves to be condemned,” Antonio Guterres told reporters at the UN headquarters in New York City on Thursday.

More than seven months since invading neighbouring Ukraine, Russia is poised to annex the four Ukrainian regions after holding referendums there over the past week that were slammed by the government in Kyiv and its Western allies as illegal.

Kremlin spokesman Dmitry Peskov said on Thursday that Russian President Vladimir Putin would sign the annexation treaties on Friday.

“I want to underscore that the so-called referenda were conducted during active armed conflict in areas under Russian occupation and outside Ukraine’s legal and constitutional framework. They cannot be called genuine expression of the popular will,” Guterres said.

He described the planned annexation of the regions as having “no place in the modern world”.

“We are fully committed to the sovereignty, unity, independence and territorial integrity of Ukraine within its internationally recognised borders,” he said, calling on Russia to de-escalate tensions.

“It is time to step back from the brink” and “end this devastating and senseless war”, he added.

Russia, one of five permanent members of the UN Security Council, is mandated to uphold the UN Charter, which prohibits the threat or use of force and champions the principle of sovereignty and territorial integrity.

US Secretary of State Antony Blinken on Thursday accused Russia of making a “land grab” and pledged again that the United States will never recognise annexation by Moscow.

“The Kremlin’s sham referenda are a futile effort to mask what amounts to a further attempt at a land grab in Ukraine,” Blinken said in a statement.

“This spectacle conducted by Russia’s proxies is illegitimate and violates international law. It is an affront to the principles of international peace and security,” he added.

Al Jazeera’s correspondent James Bays, reporting from the UN headquarters, said the UN Security Council would take action, with the US Representative to the United Nations Linda Thomas-Greenfield leading the work to present a resolution.

“We are hearing a meeting has not been scheduled yet but [the resolution] is likely to be presented on Friday,” Bays said.

According to a statement by Thomas-Greenfield on Tuesday, the resolution will be “condemning these sham referenda, calling on Member States not to recognise any altered status of Ukraine, and obligating Russia to withdraw its troops from Ukraine.”

Moscow-installed administrations in the four regions of southern and eastern Ukraine claimed late on Tuesday that voters had overwhelmingly backed joining Russia: 93 percent of the ballots cast in the Zaporizhia region supported annexation; as did 87 percent in the Kherson region; 98 percent in the Luhansk region; and 99 percent in Donetsk.

Russia has said it will see Ukrainian attacks on annexed regions as attacks on itself and will use all means to defend it.

Russian government officials have also warned that the four regions will fall under Moscow’s nuclear umbrella once they have been formally incorporated into Russia.

Russia faces ‘looming’ defeat in Lyman

The Kremlin’s move for annexation comes as Ukrainian troops move to capture the Russian-held eastern town of Lyman, threatening a new setback for Putin’s campaign in the Donbas region, comprising Donetsk and Luhansk.

The capture of the town in the north of Donetsk region could pave the way for Ukraine to make inroads into the adjacent Luhansk province, foiling Putin’s goal of seizing all of the industrial Donbas region declared after his forces failed to take Kyiv.

“The situation looks increasingly precarious for Russian forces in Lyman as Ukrainian forces are about to cut them off,” Carl Bildt, a former Swedish prime minister, said on Twitter on Thursday.

“Another painful defeat for the Russian invasion forces is looming.”

Kyiv has so far held back from disclosing details of the situation in Lyman. Russia’s defence ministry said on Wednesday that a Ukrainian offensive on Lyman had failed, with 70 Ukrainian soldiers killed.

Putin calls for correction of mobilisation ‘mistakes’

Putin said on Thursday that “all mistakes” made in a call-up to reinforce Russian forces in Ukraine should be corrected, his first public acknowledgment that the “partial mobilisation” he announced last week had not gone smoothly.

There have been widespread public expressions of discontent from officials and citizens over the way the mobilisation has been handled, including complaints about enlistment officers sending call-up papers to clearly ineligible men.

The move has also triggered protests across Russia and led to hundreds of thousands of men fleeing the country.

“In the course of this mobilisation, many questions are coming up, and all mistakes must be corrected and prevented from happening in the future,” Putin said.

“For example, I’m thinking of fathers of many children, or people suffering from chronic diseases, or who those who are already past conscription age.”

Meanwhile, Moscow is at loggerheads with Europe over leaks detected in the Nord Stream 1 and 2 gas pipelines from Russia to Germany, with still no firm indications as to what or who might have caused the damage.

Putin spoke of “unprecedented sabotage” during a telephone conversation with Turkish President Recep Tayyip Erdogan on Thursday, while top European Union officials threatened punishment for whoever was responsible but stopped short of attributing blame.

Russia has requested an urgent debate on the matter at the UN Security Council in New York on Friday.