The separatist leaders of four Russian-controlled areas in Ukraine plan to hold referendums, potentially paving the way to annex an area of the country the size of Hungary.
The self-styled Donetsk and the Luhansk People's Republics, which Russia recognised as independent before the invasion, plus the Kherson and Zaporizhzhia regions, have all asked to hold votes.
They are planning to hold them starting from Friday.
An ally of President Vladimir Putin backed the proposal from separatists covering 15 per cent of Ukraine’s territory.
Former Russian president Dmitry Medvedev said folding Luhansk and Donetsk into Russia itself would make their redrawn frontiers “irreversible” and enable Moscow to use “any means” to defend them.
Mr Medvedev, who served as Russian president from 2008 to 2012 and is now deputy chairman of the Russian Security Council, said the referendums would change the path of Russian history and allow the Kremlin more options for defence of what he said would become Russian territory.
“Encroachment on to Russian territory is a crime which allows you to use all the forces of self — defence,” Mr Medvedev said in a post on Telegram.
“This is why these referendums are so feared in Kyiv and the West.
“It is equally important that after the amendments to the constitution of our state, no future leader of Russia, no official will be able to reverse these decisions.”
Ukraine's Foreign Minister Dmytro Kuleba said “sham ‘referendums'” would not change anything.
“Neither will any hybrid ‘mobilisation’. Russia has been and remains an aggressor illegally occupying parts of Ukrainian land. Ukraine has every right to liberate its territories and will keep liberating them whatever Russia has to say,” he said in a tweet on Tuesday.
Vyacheslav Volodin, the speaker of Russia's lower house of parliament, the Duma, said he would support the folding in of parts of Ukraine that voted to join Russia.
Analysts say if Moscow formally annexes a vast additional chunk of Ukraine, Mr Putin would essentially be daring the US and its European allies to risk a direct military confrontation with Russia, thought to have the largest number of nuclear weapons in the world.
“As we've been warning for months, Russia appears to be proceeding with plans to hold sham referenda in areas of Ukraine under its control, and even in areas of Ukraine not currently under its control,” White House national security adviser Jake Sullivan said at a briefing on Tuesday.
“These referenda are an affront to the principles of sovereignty and territorial integrity,” he added.
“If this does transpire, the United States will never recognise Russia's claims.”
Russia has surrendered 10,000 square kilometres of Ukraine's territory over the past couple of weeks after a lightning offensive conducted by Ukraine.
Mr Putin on Friday brushed off the counteroffensive and cast the conflict as an attempt to prevent what he said was a western plot to carve up and destroy Russia.
Conflict in eastern Ukraine started in 2014 after a pro-Russian president was toppled in Ukraine's Maidan Revolution and Russia annexed Crimea, while Russian-backed separatists in the Donbas — which is comprised of Donetsk and Luhansk — sought to break away from Kyiv's control.
Ukraine says it will not rest until every Russian soldier is ejected from its territory.
Kyiv says it will never accept Russian control over its territory and has called on the West to supply more and better arms to fight Russian forces.
It is unclear how the separatist referendums would work during a war.
Russian and Russian-backed forces control only around 60 per cent of the Donetsk region, and Ukrainian forces are trying to retake Luhansk.
Russian forces took the entire Luhansk region earlier in the war, though Ukrainian officials said on Monday they had retaken a village in the region as part of their continuing counteroffensive.
Ukraine also still holds territory in both Kherson and Zaporizhzhia regions.
“Referendums in the Donbas are essential, not only for the systematic protection of residents of the LPR, DPR and other liberated territories, but also for the restoration of historic justice,” Mr Medvedev said.
On Tuesday it emerged Ukraine is to receive a shipment of 28 modified Soviet-era tanks from Slovenia, after a deal was struck with Germany to refill the Slovenian arsenal.
Meanwhile, UK Prime Minister Liz Truss announced that Britain would send at least as much military aid to Ukraine next year as it has this year, to the sum of about £2.3 billion ($2.6bn).
The nature of the support will be decided in response to Ukrainian needs but is likely to include more multiple-launch rocket systems, Downing Street said.
