Donetsk, Ukraine - Moscow-backed separatists in the Luhansk, Donetsk, Kherson, and Zaporizhzhia regions of Ukraine abruptly announced on Tuesday that they will hold referendums on joining Russia as soon as this week.

The separatists have been calling for the polls to be held quickly in light of a counteroffensive by Kyiv that has seen Ukrainian troops retake territory in the south and east of the country.

The United States and European Union have repeatedly denounced talk of the referendums, calling such votes a "sham" and "illegitimate."

"The United States will never recognize Russia's claims to purportedly annex any parts of Ukraine. And we will never recognize this territory as anything other than a part of Ukraine. We reject Russia's actions unequivocally," National Security Advisor Jake Sullivan said in Washington.

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He called the planned polls "an affront to the principles of sovereignty and territorial integrity that underpin the international system," saying it was clear they would be manipulated.

Three regions – Luhansk and Donetsk in the east and Kherson in the south – plan to simultaneously hold a five-day vote from Friday to September 27, the occupation authorities said.

Russian occupiers hold about 75% of the territory in Zaporizhzhia, located in the south of Ukraine. The regional capital, which had around 700,000 residents before the war, is still under the control of Ukrainian troops.

Vladimir Rogov, the head of the local military administration in Zaporizhzhia, said the referendum will only be held in places controlled by Moscow. He did not give a date for the vote beyond sometime "in the next few days."

Russia justified in large part its full-scale invasion of Ukraine on February 24 by citing the need to "liberate" Donetsk and Luhansk, which make up the Donbass region and are largely Russian-speaking.

Shortly before the attack began, Russian President Vladimir Putin recognized the independence of the two Ukrainian regions.

If Moscow were to now embrace the regions as Russian territory, the embattled separatists have suggested it could force Moscow to hit back harder against the resurgent Ukrainian forces.

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Former Russian president Dmitry Medvedev warned Tuesday that the referendums would permanently alter the region's political landscape in the Kremlin's favor.

"After they are held and the new territories become part of Russia, the geopolitical transformation in the world will take on an irreversible character," he wrote on his Telegram channel on Tuesday.

Russia could use "all means of self-protection" after the territories' accession, he added – with the clear implication of an all-out war with full mobilization.

As the war nears the seven-month mark, the Kremlin has suffered stinging defeats in the regions, with Russian troops withdrawing almost completely from the north-eastern Kharkiv region after Ukrainian counterattacks.

The Russian military in Ukraine is suffering from a shortage of personnel. The deployed professional army does not have enough resources for the war, which is still called a "special military operation" in Moscow.

However, the Kremlin hopes to mobilize the Russian population domestically, possibly even by declaring martial law or a state of emergency, analysts contend.

Russia's parliament on Tuesday fast-tracked legislative changes that could indicate possible preparation for the imposition of martial law and full military mobilization.